Release 0.3.1

[arasovic]

Documentation only, so this is a patch bump. Two commits corrected the bundled
skill since 0.3.0:

• b4ddb59 (PR docs: state that the model is chosen once per run #18) — the skill showed --model once in its command block
  but never said the choice is run-level, while step 3 spoke of preserving
  "every explicit model" and step 5 offered one to three workers. A reader
  combining those infers one model per worker and writes --model twice, which
  the CLI rejects before anything runs.
• fd19494 (PR docs: point the caller at the reason a write check skipped #19) — the run document has carried a root changes field
  since the manifest shipped, and the skill never named it. An orchestrator that
  read writes: {"skipped": "change manifest unavailable"} had no way from that
  file to learn which unavailability it hit, so it hand-verified every changed
  path instead. Step 5 also gains the condition that prevents the check from
  running: the manifest is measured against the state recorded before the first
  worker started, so an already-dirty workspace has no clean base.

No Go code changed, no JSON contract changed, and no flag was added. The release
exists because the skill ships through npm postinstall: both corrections sat on
main reaching nobody, while every session kept reading the text they fix.

Version identity

package.json, package-lock.json (both entries), and the go install line in
docs/v0-usage.md. The 0.2.0 strings in npm/test/release-metadata.test.mjs
and ARCHITECTURE.md are untouched on purpose: the first are fixtures for the
tag-versus-package-version check, the second is pi-subagent-bridge's published
version.

Local snapshot gate

Run in full at this commit, per docs/releasing.md:

• npm ci --ignore-scripts then npm run verify — exit 0
• go run ./tools/release produced the four native archives with prefix
  pi-worker_v0.3.1_, and shasum -a 256 -c checksums.txt verified all four
• npm run stage plus the staged current checkout npm pack assertion passed
• npm pack produced exactly pi-worker-0.3.1.tgz; the binary inside stamps
  v0.3.1 with this commit, and the packed skills/pi-worker/SKILL.md carries
  both corrections

After merge the release starts from the matching v0.3.1 tag push and publishes
through OIDC; no token is involved.