refactor(dev): MCP session lifecycles as Effect scoped resources (wave 3.5 stage 3, PR 2) - #159

Summary

Subsystem 1 of 4 for #152 stage 3, in the plan's order — MCP sessions first: the lifecycle seam is self-contained behind McpSessionService/McpSession, giving the smallest blast radius of the four.

• Open chain as scoped resources. McpSessionService.#openEffect acquires the epoch lease and plugin-data directory with Effect.acquireRelease inside the open scope. Releases are infallible (stage-2 lesson): failures are collected, reported through the existing cleanup-failure seam, and the last one re-raises — byte-for-byte the pre-Effect precedence. Once the McpSession is constructed it owns the resources and the scope finalizers disarm; a later failure cleans up via session.close() exactly as before.
• fix(dev): fail MCP tool calls closed when the pinned epoch vanishes #134 fail-closed contract on the typed error channel. #assertEpochCurrentEffect(): Effect<void, McpSessionStaleEpochError> probes the pinned epoch before each tool call; the mid-flight-vanish path re-probes and substitutes the typed stale-epoch failure for the incidental abort/timeout, then cancels every in-flight request and closes the session (unchanged contract, including the stderr-overflow mirror).
• Session teardown as one structured Effect. Every release step always runs in order (drain client → rm plugin data → release epoch lease → notify owner); the last failing step's error re-raises — the same last-failure-wins contract the nested finally chain had. The lifecycle serial queue is now an Effect Semaphore(1).
• Service close settles openings first, then sessions, each phase concurrently via per-element Exit, aggregating into the unchanged McpSessionServiceCloseError failure order.
• New src/effect/lift.ts: identity-preserving liftPromise/liftTry (typed dev errors and non-Error values must cross the boundary untouched).
• All Promise edges go through the stage-0/PR-1 boundary; SDK AbortSignal plumbing (session-service close abort, per-request cancel controllers) intentionally stays at the SDK edge.

Parity (all suites unchanged, zero pin flips)

• mcp-session-service.test.ts + target-mcp-runtime.test.ts (integration pool): 35 passed / 0 failed
• dev-server.test.ts, mcp-session-routes.test.ts (AB8018/409 contract), boundary tests (unit pool): 62 passed / 0 failed
• pnpm lint (incl. boundary rule) + pnpm typecheck: green
• Changeset: patch (internal rewrite).

Refs #152.