## Why
Connector declarations currently enter Codex through broad plugin
capability summaries, then MCP setup, turn tooling, and `app/list` each
reconstruct the same information. That makes executor-selected
connectors difficult to add without coupling connector behavior to the
host plugin loader.
This PR introduces a small connector-owned value that later stack layers
can populate before thread startup.
## What changed
- Move the pure app-declaration parser into `codex-connectors`,
preserving declaration order and category cleanup while leaving
host-side validation and deduplication unchanged.
- Add an immutable `ConnectorSnapshot` with ordered connector IDs and
plugin display-name provenance.
- Adapt the existing local-plugin capability summaries into that
snapshot at current consumer boundaries.
- Use the snapshot for MCP tool provenance, turn connector inventory,
and `app/list`.
- Keep the crate API narrow: no test-only snapshot accessors are
exposed.
The externally visible behavior is unchanged. Connector tools still come
from the orchestrator-owned `/ps/mcp` server, and local plugin
enablement remains owned by the existing plugin loader.
## Stack scope
This is the foundation only. It does not read selected executor packages
or change thread startup. #29852 adds the executor-backed declaration
reader, and #29856 composes selected declarations into a thread
snapshot.
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