feat(web): add bounded transcript content search

[testikun]

Problem

Issue #349 requires transcript-aware server search, but the current Web snapshot exposes only a bounded title/cwd/first-message projection. The open metadata-search PRs do not provide a safe mechanism for scanning Session JSONL content or tool evidence.

Relates to #349. This PR implements the transcript-scanner slice only; it does not duplicate PR #362's endpoint or PR #376's pagination work.

Value

• Gives the Web host a reusable server-side primitive for finding prior user/assistant text and bounded tool evidence.
• Keeps Pi Session files authoritative while making partial, malformed, changed, unavailable, and budget-limited outcomes observable.
• Preserves exact Session/message/turn/workspace/source/timestamp provenance without retaining an unbounded index.

Approach

• Add searchWebTranscripts() over a caller-authorized Session catalog; the scanner never discovers or widens filesystem authority.
• Enforce hard caps for catalog files, total bytes, per-file bytes, line bytes, retained results, snippet bytes, and wall time; support AbortSignal cancellation.
• Require regular non-symlink files within canonical allowed roots and exact workspace membership, then validate the Session header identity before publishing matches.
• Scan JSONL incrementally in fixed-size chunks, retain only a bounded newest-result set, propagate turn identity across intervening Pi entries, and re-check file identity after scanning.
• Search user/assistant text plus tool call/result evidence, while excluding private result details. Return control-safe, bidi-safe, UTF-8-bounded plain-text snippets with explicit truncation evidence.
• Fail soft per file for malformed, changing, deleted, oversized, and unauthorized inputs, with exact partialReasons and accounting.

The existing lightweight metadata filtering remains separate. Endpoint composition, pagination/cursors, and derived index invalidation remain follow-up slices of #349.

Validation

• node --test --experimental-strip-types tests/web/transcript-search.test.ts — 8 passed
• bun run check — passed
• bun run test — Node: 1249 passed, 0 failed, 1 platform skip; Vitest: 30/30 passed
• Test coverage includes a real Pi-written Session file, user/assistant/tool evidence, provenance, Unicode snippets, control/bidi stripping, malformed JSONL, archive filtering, root/workspace/symlink rejection, header mismatch, every budget, timeout, and cancellation.
• Manual UI validation: not applicable; this PR adds no UI behavior.

Impact

• User-visible behavior: None until a Web endpoint/UI composes the scanner.
• Model-visible context/tools: None.
• Runtime/lifecycle: Adds a read-only, cancellable, bounded Session-file scanner; no second Session store or index.
• Persisted config/data: None.
• Compatibility/risk: Additive module only. The caller must supply the authorized Session catalog and roots; the scanner fails closed on provenance or path mismatches.

[testikun]

CI status note: Node 22/24 completed this PR's repository checks and failed only at the final fresh-install Smoke-test packed standalone Web CLI step, where the child exits before the test can read its URL. The same failure now occurs on unrelated PRs #385, #386, and #387.

I reproduced the packed install independently: npm now admits Pi 0.85.0 through OpenPI's published peerDependencies: "*"; the CLI then exits with Cannot find module '@earendil-works/pi-server'. The checkout/lock-based bun run check and full test suite remain green. I recorded the shared packaging regression and suggested peer-range follow-up in #328: #328 (comment)

No dependency workaround is mixed into this feature PR.