Resume MCP executions after session reinitialization

[SpaceMatthieu]

Summary

Allow model-mode MCP clients to resume a paused execution after they initialize a new session. The in-process session store now finds the owning live engine only when account, organization, MCP resource, and approval mode match; it preserves caller-side write authorization, concurrent resume joins, and settled-result replay.

This intentionally remains an in-memory fix: process restarts and normal session expiry still discard resumable execution state.

Linked issue

Fixes #1778

Verification

• bun run format:check — passed across 2,158 files.
• bun run lint — passed across 1,878 files with no warnings or errors.
• bun run typecheck — all 45 packages passed.
• bun run test — all 39 non-e2e package tasks passed, including 264 MCP-host tests and 18 execution-engine tests.
• e2e — no dedicated repository e2e scenario was run; the MCP session-store suite covers the cross-session protocol flow, authorization boundaries, concurrent joins, settled replay, failure handling, and session lifetime.

Checklist

• Added a changeset (bun run changeset).
• Added or updated tests for the new behaviour.
• No secrets, credentials, or private data in the diff.