guides: add SEAT-IDENTITY and FLEET-COMMS; harden CODE-REVIEW evidence rules

Three guides that existed only as one host's working copy, promoted to framework
templates so every estate gets them. A working copy under ~/.mosaic binds one
host; only a template here binds all of them.

SEAT-IDENTITY.md (new) documents how a seat's git credential is actually
resolved after #1311: identity from MOSAIC_GIT_IDENTITY, then
mosaic.gitIdentity, then the stdin username; host mapped to a store prefix; then
ONE of two stores chosen by whether the seat directory exists, with no
precedence and no fallback between them. A seat with a directory and an empty
slot fails closed rather than reaching the service store, and that is the point.

It also corrects how to find the helper. credential.helper commonly names an
absolute path, so `command -v git-credential-mosaic` answers a different question
than the one git asks, and the two stop agreeing the moment the PATH copy is
removed. Git also tries EVERY configured helper in order, so a fail-closed helper
in front silently hands the request to whatever is configured behind it. The
guide says to read the whole list.

FLEET-COMMS.md (new) documents agent-send.sh: the class table, the addressing
preamble, and the exit codes — including that rc=2 means the text reached the
pane as an unsubmitted draft, so retrying double-sends it. Confirm with
capture-pane instead. It also says to measure the fleet rather than trust
roster.yaml, which on a live host was simultaneously naming a socket that did not
exist, listing seats that were not running, and omitting seats that were.

CODE-REVIEW.md gains an Evidence Discipline section: a green is not a result
until you have shown it could go red, measurement and explanation are separate
sentences, verify by content on the ref that ships rather than by ancestry of a
local sha, and confidence is part of a finding. Plus four shell-measurement rules
earned on #1311, each of which produced a wrong conclusion first — `cmd | tail;
echo rc=$?` reports tail's status, a missed glob under pipefail exits 2 and kills
the run under set -e, nonzero-with-no-output is an environment question before it
is a code question, and `git -C` in a non-repo directory answers from the
enclosing repo.

The estate-specific repository exception that lived in the working copy is not
carried here. The template says an estate may document one, scoped to a named
repository and never precedent for a second.

Both new guides are added to the two routing tables that agents read.

packages/mosaic/framework/defaults/AGENTS.md

@@ -43,6 +43,8 @@ overwritten on upgrade. (Layer model: `constitution/LAYER-MODEL.md`.)
 | Secrets / vault usage                              | `guides/VAULT-SECRETS.md`          |
 | Tool/credential reference (service CLIs, wrappers) | `guides/TOOLS-REFERENCE.md`        |
 | Memory protocol (OpenBrain capture/recall)         | `guides/MEMORY.md`                 |
+| Seat identity, git credentials, token slots        | `guides/SEAT-IDENTITY.md`          |
+| Reaching another agent (fleet comms)               | `guides/FLEET-COMMS.md`            |
 
 ## Subagent Model Selection (Cost — Hard Rule)