feat(db): enforce input-scaled chunking invariant + PR #55 deferred findings (#76)

[edheltzel]

Summary

Consolidates the non-blocking deferred findings from the PR #55 review (issue #76) into one surgical change set, and turns the chunk.ts audit note from prose into an enforced invariant.

The guard test surfaced a real, latent post-audit bypass: bumpAccess (frecency, issue #153, added after the 2026-06-10 audit) built an input-scaled id IN (...) list with Array.from(ids, () => '?') and never chunked it. A high---limit recall could push that bind count past the historical SQLite variable limit on stricter platforms. Brought it into line with the existing chunked() pattern (same as aging.ts / dedup.ts), inside the existing transaction so the bump stays all-or-nothing.

Findings addressed

1. Guard test for the audit note (tests/lib/chunk-audit.test.ts) — scans src/ + hooks/ for the input-scaled placeholder idiom (() => '?', covering both .map(() => '?') and Array.from(x, () => '?')) and fails when a site bypasses chunked() outside a fixed-size allowlist. Comments are stripped first so doc mentions of the idiom don't self-trigger. Includes a non-vacuous-detection test.
2. hooks/ carve-out — chunk.ts audit note now documents that hooks can't import the helper (self-contained-hooks rule) and must inline a local equivalent; a one-line Key Conventions entry in AGENTS.md puts the invariant where the next SQL author will see it. consolidate-core.ts's fixed-column VALUES is the one allowlisted site.
3. @throws {RangeError} JSDoc on chunked(), noting chunked([], 0) throws (size validates before the empty-input check).
4. FOR_OPENCODE.md rule 5 drift — restores the "confirm with the user before dumping credentials" clause the other three guides carry.

Audit note also refreshed: the input-scaled paths are now dump.ts, aging.ts, dedup.ts, and memory.ts — all routed through chunked() and enforced by the new test.

Out of scope (flagged for follow-up issues)

The review's "follow-up issue candidates" (production busy_timeout, the 4-worker race test's missing results.length === 4 assertion, parent_loa_id cycle recursion) and the deleteSession end-to-end test are pre-existing, larger concerns — left for their own tracked issues rather than expanding this change set. (The embeddings-orphan item already shipped via #46.)

Verification

• bun run lint (tsc --noEmit) clean.
• bun test — full suite 1178 pass / 0 fail.
• Proved the guard is not vacuous against real source: temporarily un-chunked bumpAccess and confirmed the guard test fails naming src/lib/memory.ts with a remediation message; passes once restored.

Closes #76