git credentials: fail closed, and read a seat's token from its own slot

Two changes to one rule: a credential is resolved from exactly one place,
and an identity that cannot be resolved is refused rather than substituted.

FAIL CLOSED. Both readers ended in an unconditional fall-through to the
shared Gitea account whenever an identity did not resolve. Every seat in a
fleet therefore pushed, opened PRs and filed reviews under one account, and
a record made that way cannot be traced to the agent that made it
afterwards. The fallback now applies only where there is no attribution to
lose: a host with no fleet. Where seats exist, an unresolvable request emits
nothing, exits nonzero, explains itself on stderr, and — in the git helper —
appends a record naming the identity, host, reason and cwd, and no token
value, to ${MOSAIC_CREDENTIAL_SPOOL:-~/.local/state/mosaic-credential-escalations}.

A host runs a fleet when <brain>/fleet/agents exists, which is the signal
packages/mosaic/src/fleet/brain-home.ts already uses to decide a brain is
active, resolved the same way (MOSAIC_BRAIN_HOME, else ~/.mosaic). This is
what keeps the change a no-op for an operator who has not provisioned
per-slot tokens: no fleet directory, shared account, unchanged. It is also
why there is no environment variable to restore the old behavior — one would
reintroduce the substitution being removed.

STORE SELECTION. Both readers hardcoded ~/.config/mosaic/secrets/gitea-tokens,
so a seat's own secrets/ slot was invisible to the framework: a seat could
hold a valid credential and still be served the shared account. The store is
now chosen by what the identity is. An identity with a directory under
<brain>/fleet/agents/ is a seat and is read only from
<brain>/fleet/agents/<id>/secrets/; any other identity is a service identity
and is read from the framework store. There is no precedence between them
and no fallback from one to the other, so a seat with an empty slot is
refused even when a same-named token sits in the framework store. Two copies
of one credential are drift rather than redundancy, and drift surfaces as
the stale copy returning 401, which reads as a revoked token and sends
whoever debugs it somewhere else.

detect-platform.sh is in scope alongside git-credential-mosaic because they
are the two readers of these tokens. Patching only the git helper would make
"one credential, one location" true for push and fetch and false for
pr-create.sh, issue-create.sh and pr-review.sh, which is the harder failure
to notice.

TESTS. The three assertions that pinned the shared-account fall-through are
now fail-closed assertions, and a refusal is checked four independent ways:
nonzero exit, empty stdout, a stderr diagnostic naming identity and host,
and no shared token value anywhere in the output. The exit code alone would
pass against a helper that emitted the credential and then failed. Added:
seat-slot resolution, the no-cross-store-fallback case with a control
proving the framework-store file it declines to read is readable, no-identity
on a fleet host, the fleet gate firing on the default ~/.mosaic and not only
on an injected MOSAIC_BRAIN_HOME, and a cross-host leak check. Both suites
were run against the pre-change code as a control and fail there on exactly
the shared-token emission.

shellcheck is not installed on the authoring host, so the rewritten helper
is unlinted locally and CI is the first lint of it.

packages/mosaic/framework/tools/git/README.md

@@ -43,7 +43,7 @@ fleet commits, pushes, and opens PRs under one identity — with no cryptographi
 separation between an author and a reviewer.
 
 Both `git-credential-mosaic` and `get_gitea_token()` resolve an optional **per-agent
-identity** before falling back to the shared account:
+identity**:
 
 1. `MOSAIC_GIT_IDENTITY` environment variable, or
 2. `git config --get mosaic.gitIdentity` (set per-worktree; persists on disk across
@@ -51,12 +51,54 @@ identity** before falling back to the shared account:
 3. (git-credential-mosaic only) the username git itself supplies for the credential
    request.
 
-If the resolved identity has a token file at
-`~/.config/mosaic/secrets/gitea-tokens/gitea-{usc,mosaicstack}-<agent-id>.token`, that
-identity + token is used. **Nothing configured → nothing changes**: with no per-slot
-token file present, both tools fall through to the existing shared-account path
-unchanged, so this feature is a no-op on any host that hasn't provisioned per-slot
-tokens.
+### Which store a credential is read from
+
+The store is chosen by what the identity **is**, not by which file happens to exist first:
+
+| The identity | Its credential is read from |
+| --- | --- |
+| has a directory at `<brain>/fleet/agents/<id>/` — it is a **seat** | `<brain>/fleet/agents/<id>/secrets/gitea-{usc,mosaicstack}-<id>.token` |
+| does not — it is a **service identity** | `~/.config/mosaic/secrets/gitea-tokens/gitea-{usc,mosaicstack}-<id>.token` |
+
+`<brain>` is `MOSAIC_BRAIN_HOME` if set, else `~/.mosaic` — the same resolution
+`packages/mosaic/src/fleet/brain-home.ts` performs.
+
+**There is no precedence between the two stores and no fallback from one to the other.**
+A seat whose slot is empty is refused even when a same-named token sits in the framework
+store. One credential lives in exactly one location: a second copy is drift rather than
+redundancy, and the way drift surfaces is a stale copy returning 401, which reads as a
+revoked token and sends whoever debugs it to the wrong place.
+
+### What happens when nothing resolves
+
+| identity resolves | token in its store | host runs a fleet | result |
+| --- | --- | --- | --- |
+| yes | yes | — | that identity + token |
+| yes | no | — | **fail closed** |
+| no | — | yes | **fail closed** |
+| no | — | no | shared account, unchanged |
+
+A host "runs a fleet" when `<brain>/fleet/agents` exists — the same signal `brain-home.ts`
+uses to decide a brain is active.
+
+Failing closed means: nothing is emitted, the exit status is nonzero, a stderr diagnostic
+names the identity, its source, the store it resolved to and the path that was expected,
+and `git-credential-mosaic` additionally appends a record (identity, host, reason, cwd —
+never a token value) to `${MOSAIC_CREDENTIAL_SPOOL:-~/.local/state/mosaic-credential-escalations}`.
+The git operation fails; nothing is attributed to anyone.
+
+The shared-account fallback that used to cover these two cases is why a PR could be
+authored, commented and merged under an account whose owner did not open it — every seat
+shared one identity, so the record could not be traced back afterwards. An
+under-provisioned agent is refused rather than handed the most privileged account
+available.
+
+**On a host with no fleet, nothing changes**: no `fleet/agents` directory means the shared
+account still answers, so this is a no-op for an operator who has not provisioned per-slot
+tokens. On a host that does run a fleet, a human doing manual git work needs an identity
+of their own — `MOSAIC_GIT_IDENTITY=<id>` with a provisioned slot. There is deliberately no
+environment variable that restores the fallback; one would reintroduce exactly the
+substitution this removes.
 
 ### Enabling it for a clone