feat(harness): add client context reconstruction

[chughtapan]

What changed

• add a package-private client reconstruction engine over the typed search_conversations and read_conversation capabilities
• keep current-conversation content exactly equal to the live MCP turn batch while reconstructing only cross-conversation history
• persist stable source checkpoints per target conversation through an injected Effect KeyValueStore
• keep search/read page cursors local to one drain and leave prior checkpoints unchanged when reconstruction fails
• add focused coverage for pagination, target/source checkpoint independence, later live observations, cyclic cursors, and failed page reads

Why

This is the next production-first HarnessClient slice. It composes the MCP read plane with live replyable observations without choosing the still-deferred public context-entry projection, filesystem layout, or adapter contract.

Reading the live target conversation is intentionally excluded: a history read could race ahead of the notification and attach a later message to the wrong reply opportunity.

Impact

There is no new package export or public HarnessClient field yet. The engine is a private integration seam for the later turn-projection and adapter migration slice.

Stacked on #944. Foundations: #943, merged #942, and upstream simplification #924.

Validation

• pnpm nx run @moltzap/client:test — 31 files, 226 tests passed
• pnpm nx run @moltzap/client:typecheck:tests
• pnpm nx run @moltzap/client:lint
• pnpm nx run @moltzap/client:arch:check
• pnpm nx run workspace:precommit — 61 tasks passed with normal Nx caching
• independent code audit and post-fix re-audit: clean

[chughtapan]

Superseded by #974, which collapses this stack into one PR against main. This branch's commits are all contained in it — nothing is lost, and this thread stays readable. Closing to leave one place to review.