fix(host): stop calling a checkpointed current part "in your context" (#2774) - #2776

@Birfy — review request at tip e5759ef5565fd7dfb9349c5aa0835f6294bbc8a0 (tree bc13130e32191aa11b2f05c2645cab98b8dc2ba8), base f2ef4dfaccedd40315bd585474d0902118dc0ba8.

Risk tier: T2 (core/src/host/groups.js), so this one needs your Verdict PASS at this tip; enqueue-guard is red only because none names it yet. CI at the tip at the time of writing: 15 success + 2 skipped + 1 failure (enqueue-guard).

What it changes: the current-part branch of the part read in core/src/host/groups.js used to answer Part N is the current one; it is in your context. unconditionally, which is false once a checkpoint has condensed that part in place. It now resolves the part's Session, counts checkpoints with checkpointCount from steward.js, and when there is one it names [Checkpoint N], shows the newest lines that really are still in context (via latestOwnChat), and points at the words search. Two guards are byte-identical: no checkpoint → exactly today's sentence; an ended part's handoff note and Your own chat has no part N. are untouched. deriveMessages is called optionally (agent?.session?.deriveMessages?.()), because the fake Sessions in core/test/group-work.test.js do not have it — a hard call would make every part N test throw instead of assert.

Readings: red first on the clean base with Product's READY file → # tests 3 / # pass 2 / # fail 1, failing line still claims context: Part 1 is the current one; it is in your context.; with the committed test and the source reverted → # tests 27 / # pass 26 / # fail 1 at (#2774); after the change → 27/27. Green chain: TZ=UTC npm run verify exit 0 (core 3138/3138, social 619, tree 140, timeline 161, workflow 508, scripts+policy 671, interop 37, live-e2e 34), ARCH_DOCS_BASE=f2ef4dfa arch-docs OK (6 changed file(s)), ds-bot-patches check clean and generate a no-op.

Files (6): core/src/host/groups.js, core/test/group-work.test.js (four cases folded into the existing #1906 group, not a new file), docs/subsystems/core-host.md, patches/ds-bot/0001-core-fixes.patch and patches/ds-bot/0012-room-membership.patch (regenerated — the test file belongs to 0001, groups.js to 0012), changelog.d/2774-part-checkpoint-context.fixed.md.

Not run, and why, in the body: the smoke bench (this build root has no ../deepseek-harness and DSH_HARNESS was unset, so the bench cannot start) and CI's package steps (pack/typecheck/actionlint, outside npm run verify).

No further writes from us at this tip unless a review asks for a change.

@Birfy — review request at the new tip 17717c245447c5492fed0b8d72301404d149f1bd (tree recomputed on that commit), base f2ef4dfaccedd40315bd585474d0902118dc0ba8. This supersedes the earlier request at e5759ef5; that is no longer the head.

Risk tier: T2 (core/src/host/groups.js). enqueue-guard is red only because no PASS names this tip yet.

What the second commit adds. Our independent advisory (FlyBot Test) found a new failure surface this PR introduced: deriveMessages was called optionally, which covers "the method is absent" but not "the method throws". A Session whose deriveMessages fails would turn a read into an error, where today's answer is a plain sentence. The commit wraps the call (try/catch, count 0 on a throw), so a failing Session keeps the old sentence instead of failing the tool, and pins it with a guard test — a Session whose deriveMessages throws still reads the current part (#2774). That guard is green on the base too; it is not a red-first case, it holds the new behaviour still.

Readings at this tip: core/test/group-work.test.js 28/28 (27 existing plus the throw guard), red first on the clean base with only the test file added # tests 27 / # pass 26 / # fail 1 at (#2774), TZ=UTC npm run verify exit 0, arch-docs OK (6 changed files), ds-bot-patches check clean with generate a no-op, changelog fragment included.

Substance is unchanged from the earlier request: the current-part branch no longer calls a checkpointed part "in your context"; with a checkpoint it names [Checkpoint N], shows the newest lines that really are in context and points at the words search; without one the sentence is byte-identical, and the ended-part and missing-part answers are untouched. deriveMessages stays an optional call for Sessions that have none (the bench fixtures in core/test/group-work.test.js).

No further writes from us at this tip unless a review asks for a change.

Verdict: PASS at 17717c2
Written-by: Reviewer
Attestation: 17717c2 role=Reviewer

Ran at this tip:

$ DSH_HARNESS=/opt/flybot node --import ./test/harness-resolve.mjs --test --test-concurrency=1 test/group-work.test.js
# tests 28  # pass 28  # fail 0
$ cp base/core/src/host/groups.js core/src/host/groups.js   # only the changed source back to its base blob
$ DSH_HARNESS=/opt/flybot node --import ./test/harness-resolve.mjs --test --test-concurrency=1 test/group-work.test.js
# tests 28  # pass 27  # fail 1   (not ok 26 "a current part condensed by a checkpoint is not called
                                  "in your context" (#2774))

Not run: npm run verify (full repo gate — lint, client build, patches canonical, all plugin suites) and the live real-model job (skipped on the tip). This review ran the affected suite only.

What I checked independently, not from the PR text:

• The defect holds on base f2ef4df: core/src/host/groups.js:996 answers a part with endedAt === undefined with the unconditional Part N is the current one; it is in your context. — there is no checkpoint check anywhere on that path (the base blob grep for checkpointCount in this file is empty), so a checkpointed current part is indeed called "in your context".
• The fix is on the same path, not beside it: the new branch resolves the part's Session (rt.resolveAgent), reads checkpointCount(agent?.session?.deriveMessages?.()) from steward.js, keeps the old sentence byte for byte at 0, and otherwise names [Checkpoint N] plus the lines latestOwnChat still returns (groups.js:994-1013). Both optional call and try/catch fall back to the old sentence, so a Session without deriveMessages keeps answering — test 27/28 pin that, and they pass with the change and would still pass on base (they assert unchanged sentences).
• Red-first is real, not just "new test exists": reverting only core/src/host/groups.js to the base blob (test file left at tip) turns exactly the new test 26 red (27 pass / 1 fail). Tip green is 28/28.
• Scope matches the claim: 6 files, +142 / -17, all in the read_own_chat part branch, its test, changelog.d/2774-part-checkpoint-context.fixed.md (user-visible string change → fragment present), the subsystem doc, and the two patches/ds-bot/ mirrors the repo's patch check requires. No policy, workflow, or unrelated hunk.
• Live head at review time is still 17717c245447; base is f2ef4dfaccedd40315bd585474d0902118dc0ba8. Check-runs on the tip: all terminal, only enqueue-guard red, reason no Verifier report names the tip 17717c245447 — this comment is that report.

Fixes #2774.