feat(mcp): add uninstall + --check + README section for MCP pipeline

Three coupled additions close the MCP pipeline thread.

mcp/install.py grew --uninstall and --check modes via argparse. The
default install behavior is unchanged.

--uninstall iterates over servers.json and runs `claude mcp remove
<name> -s user` for each, skipping anything not registered. Idempotent.

--check is the dry-run drift report. For each server, classify as ok
(in both servers.json and `claude mcp list`), MISSING (configured but
not registered), or EXTRA (registered but not in servers.json). Exit
non-zero only on MISSING since EXTRA entries are often deliberate (the
claude.ai web servers register out-of-band). Smoke test against the
live config: 9 ok, 0 missing, 3 EXTRA, exit 0.

Two new Makefile targets:
- make uninstall-mcp invokes the --uninstall mode.
- make check-mcp invokes the --check mode.

README.org gained an MCP section under Two install modes covering all
three targets, the OAuth-token-on-disk story, and a pointer to
mcp/README.org for the full pipeline.

Closes TODO #7 (uninstall + --check) and TODO #8 (README MCP section).

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