fix(memory-ingest): switch writer to gbrain put, retire legacy put_page

[AZ-1224]

Summary

bin/gstack-memory-ingest.ts calls the legacy flag-based gbrain put_page --slug X --title Y --type Z --tags T form. gbrain v0.27 retired it for the positional put <slug> (content via stdin, YAML frontmatter for metadata). Against any current gbrain, every page errors [put-error] ... Unknown command: put_page and bulk ingest produces 0 successful writes.

This PR:

• Rewrites gbrainPutPage to invoke gbrain put <slug>, injecting title/type/tags as YAML frontmatter when the page body does not already start with one. The transcript builder already prepends frontmatter; the writer-side injection only fires for raw artifact pages.
• Bumps gbrainAvailable() to probe gbrain --help for the put subcommand. If a future gbrain renames or removes it, the script fails fast with one clean error rather than N copies of "Unknown command: ...".
• Adds two regression tests in test/gstack-memory-ingest.test.ts that stand up a fake gbrain shim on PATH:
  1. Writer hits gbrain put <slug> with stdin body + frontmatter and never invokes put_page.
  2. Pointing the writer at a legacy-only shim surfaces a single missing-put error from the availability probe.

Scope is intentionally tight: one function rewrite, one availability probe, two tests. No refactor of the writer abstraction.

Repro (before the fix)

# With gbrain v0.27.0+ on PATH and any transcripts in ~/.claude/projects/.../*.jsonl:
bun bin/gstack-memory-ingest.ts --bulk
# Every page errors: [put-error] ... Unknown command: put_page

Verified

• bun test test/gstack-memory-ingest.test.ts — 17/17 pass (15 existing + 2 new regression tests).
• Real-world: 61/62 pages ingested cleanly post-patch on a v0.27.0 install. The single failure was a within-batch slug collision unrelated to the writer.

Notes

• The V1.5 NOTE block at the top of the file referenced put_page in prose. Updated to describe the current gbrain put <slug> (stdin, frontmatter) shape so the doc stays accurate; the future put_file blob-routing TODO is unchanged.
• Section comment // ── Writer (calls gbrain put_page) ── updated to // ── Writer (calls \gbrain put`) ──`.

Test plan

• bun test test/gstack-memory-ingest.test.ts passes
• Manual --bulk run on a real gbrain v0.27.0 install ingests pages successfully
• Fake-shim test catches a regression to put_page (verified: rolling back the writer makes the new test fail)

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