[Birfy]

fix(memory): cap the memory journal so it keeps its newest changes

Fixes #2807

Owner: Dev
Reviewer: Birfy
Written-by: Dev
Bot: Dev
Risk tier: T2
Base: f42ad01
Claim-Id: 3f049f868ab3ff391e0301d6dc1aea19

What changed

<home>/memory/journal.jsonl was append-only and never pruned: every remember, replace, forget and archive added a line, and nothing ever removed one. The memory itself is at a steady state, so only this side channel grew without bound.

core/src/host/memory.js now keeps the newest changes under two caps, JOURNAL_KEPT (500 lines) and JOURNAL_MAX_BYTES (1 MiB), the same shape as the state journal's pruneEntries: past a cap the oldest lines are folded into one {"time":…,"op":"truncated","dropped":N} record, an earlier fold's count carries into the next one, and the whole file is rewritten through a temp name and rename (replacePrivateFile, the target's mode kept) so a reader never sees half a line. A trim keeps 90% of the cap, so the file is rewritten every few changes rather than on every one. The normal log lines and mode: PRIVATE_FILE_MODE are unchanged; journal.jsonl still has no reader outside the tests.

core/test/memory-journal-cap.test.js is new: a real apply() host runs 300 remember+forget turns (600 writes, MEMORY.md back to its 20-byte skeleton), then asserts the file is at most 1 MiB and 500 lines, that the oldest note is gone and one truncated record names the fold, and that a change made after the prune is still the newest line. The regenerated patches/ds-bot/ mirrors are in this diff; changelog.d/2807.fixed.md records the user-visible change.

Red first

At base f42ad016, with only the new test applied and memory.js as it was:

$ cd core && TZ=UTC node --import ./test/harness-resolve.mjs --test test/memory-journal-cap.test.js
not ok 1 - the memory journal keeps its newest lines, like the state journal
  error: 'the journal keeps at most 500 lines (it keeps 600)'
# tests 1 / # pass 0 / # fail 1

600 lines is the defect: the 300 add+forget cycles left every line behind while the memory was empty again.

Ran at this tip

$ cd core && TZ=UTC node --import ./test/harness-resolve.mjs --test test/memory-journal-cap.test.js
# tests 1  # pass 1  # fail 0
$ cd core && TZ=UTC node --import ./test/harness-resolve.mjs --test test/memory*.test.js
# tests 134  # pass 134  # fail 0
$ TZ=UTC npm run verify            # repo root, exit 0
core: # tests 3198 / # pass 3197 / # fail 0 / # skipped 1
social 619/619, tree 145/145, timeline 161/161, workflow 528/528
.github/scripts + policy 673/673, interop 37/37, live-e2e 34/34
core lint: Found 88 warnings and 0 errors; arch-docs: OK (5 changed file(s))
build-client --check green; 10 patches apply cleanly; core/ equals ds-bot/ + patches/ds-bot/
canonical: patches/ds-bot/ is what generate writes (generate is a no-op)
$ node .github/scripts/changelog.mjs check
changelog: 25 fragment(s) in changelog.d/ are well formed

Not run

• The smoke test: not part of npm run verify; it needs the test bench with a harness checkout, and this change is covered by the core suite above.
• npm run pack, the typecheck and actionlint: the CI package job runs them; they are not part of npm run verify.

Docs-not-needed: the retention bound sits inside the core-host page's existing description of the memory journal (file checks, write path, side channel); it adds no folder, no API and no new reader.

Checkpoint

{
  "pr": 2809,
  "claim": "Dev",
  "issue": 2807,
  "summary": "The memory journal was append-only: 300 remember+forget turns left 600 lines while MEMORY.md was back to 20 bytes. `memory.js` now keeps the newest 500 changes (1 MiB) and folds the rest into one `truncated` record, rewritten through a temp name like the state journal. A new real-host test reproduces the growth and pins the cap, the fold count and the newest-line rule; regenerated `patches/ds-bot/` mirrors and a changelog fragment ride along.",
  "files": [
    "core/src/host/memory.js",
    "core/test/memory-journal-cap.test.js",
    "patches/ds-bot/0001-core-fixes.patch",
    "patches/ds-bot/files/test/memory-journal-cap.test.js",
    "changelog.d/2807.fixed.md"
  ],
  "checks": {
    "ran": "TZ=UTC node --import ./test/harness-resolve.mjs --test test/memory-journal-cap.test.js (1/1); test/memory*.test.js (134/134); TZ=UTC npm run verify (core 3197/3198, social 619, tree 145, timeline 161, workflow 528, scripts+policy 673, interop 37, live-e2e 34; arch-docs OK; canonical no-op)",
    "result": "green",
    "at": "bf6ccf943e3024c3b9e7adb89f72df2cc262a8f7"
  },
  "next": "none — ready for review"
}