fix(auth): mosaic-managed auth lives under the data root, never ~/.pi (#48)

Owner direction: the stack must never impact default harness usage.
Correction to M19 as shipped (nothing had been created in ~/.pi — the
move breaks nothing).

- Mosaic-managed accounts: <dataRoot>/auth/<account>.json, perms 0600
  enforced (loose perms flagged in listings, refused by --auth — mirrors
  gitea-api.sh credential hygiene).
- ~/.pi is read-only to the stack, permanently; the only interaction
  remains the existing read-only container mount of the default
  credential. Recorded as a ROADMAP standing decision.
- auth.sh is now config-driven (data root from config.json, fail closed,
  consistent with every other tool); status reports both sources labeled.
- agent.sh --auth resolution moved after load_config (needs the data
  root); missing/symlinked/non-0600 accounts refuse.
- test-auth.sh: 15 no-Docker cases (accounts-create-nothing, loose-perms
  refusal, invalid-config refusal added). Test-authoring correction
  recorded in BUILD-LOG (fixture-state mismatch caught before running).

Suites 24/15/90/14/17 + verify green.

docs/TOOLS.md

@@ -65,14 +65,21 @@ provider; resolution order `--api-key` → `auth.json` → env → models.json).
 No credential material is ever printed — provider names, credential types,
 and env var NAMES only.
 
+Ownership rule (#48): `~/.pi` is read-only to the stack, permanently. The
+only interaction is the existing read-only container mount of the default
+credential (`PI_AUTH_FILE`, default `~/.pi/agent/auth.json`). Mosaic-managed
+accounts live under the data root: `<dataRoot>/auth/<account>.json`, perms
+0600 (mirroring `scripts/gitea-api.sh` hygiene — loose perms are flagged in
+listings and refused by `--auth`).
+
 | Command | Purpose | Notes |
 |---|---|---|
-| `scripts/auth.sh status` | Per-provider credential report | Types (`oauth`/`api_key`), perms, env-side names (informational); exit 3 when the file is missing |
-| `scripts/auth.sh accounts` | List named account files | `auth.<account>.json` beside the credential file; marks the active one |
+| `scripts/auth.sh status` | Report both credential sources | Default harness credential (read-only) + mosaic-managed accounts; never prints material |
+| `scripts/auth.sh accounts` | List mosaic-managed accounts | Under the data root; marks the active one; flags non-0600 |
 
-The file is the compose read-only mount source (`PI_AUTH_FILE`, default
-`~/.pi/agent/auth.json`); named accounts ride the same mount per launch via
-`agent.sh --auth`. Headless task runs keep the default credential.
+`agent.sh --auth <account>` injects `<dataRoot>/auth/<account>.json` as the
+launch's `PI_AUTH_FILE`; missing/symlinked/non-0600 accounts refuse.
+Headless task runs keep the default credential.
 
 ## Release