fix(mcp): accept exact thread sync selectors

[morluto]

Description

The MCP contract documents exact thread synchronization with selection: "threads", but schema defaults and application validation treated omitted repository-only fields as if the caller had supplied them. Valid exact selectors were rejected before the sync job ran.

This moves mode-specific defaults to runtime, rejects repository filters only when they are actually present, and validates every exact thread reference before job submission. Repository grouping still shares bounded GitHub requests, while the durable result expands back to one ordered outcome per requested thread. Repeated identical observations no longer add duplicate history rows.

The stdio regression sends the documented selector through the real MCP transport and proves one exact thread is synchronized without a broad list read.

Closes #112.

Testing

• make verify
• Focused MCP stdio, schema, ordering, and idempotent-observation regression coverage
• Storage and side-effect invariants remain intact
• No unrelated cleanup or generated-output churn is included

Compatibility and scope

Repository-selection defaults remain unchanged. Exact-thread mode performs bounded GitHub reads only after explicit tool invocation; no GitHub mutation path is added.

[chatgpt-codex-connector]

You have reached your Codex usage limits for code reviews. You can see your limits in the Codex usage dashboard.