fix(cli): stop serve --mcp from confusing humans — hide it + explain on a TTY (#867)

`codegraph serve --mcp` is the stdio MCP server an AI agent launches for itself
(the installer wires it into every agent's MCP config), not a command a human
runs. Run by hand in a terminal it just hung waiting for JSON-RPC, looking
broken.

- Hide `serve` from `--help` (commander `{ hidden: true }`); it stays fully
  invocable, so agents are unaffected.
- When stdin is an interactive TTY (a person — never the agent's pipe or the
  detached daemon), print what it is and point to `codegraph status` /
  `codegraph daemon`, then exit instead of hanging.
- README: drop `serve --mcp` from the CLI Reference and stop the troubleshooting
  section from telling users to run it; keep the accurate "your agent launches
  it" note.

Verified: agent path intact (22 MCP handshake/daemon tests pass), `serve` absent
from --help, and the TTY path prints the message and exits cleanly.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -485,7 +485,6 @@ codegraph callers <symbol>        # Find what calls a function/method (--limit,
 codegraph callees <symbol>        # Find what a function/method calls (--limit, --json)
 codegraph impact <symbol>         # Analyze what code is affected by changing a symbol (--depth, --json)
 codegraph affected [files...]     # Find test files affected by changes (see below)
-codegraph serve --mcp             # Start MCP server
 codegraph daemon                  # Manage background daemons — pick one to stop (alias: daemons)
 codegraph telemetry [on|off]      # Show or change anonymous usage telemetry
 codegraph upgrade [version]       # Update to the latest release (--check, --force)
@@ -723,7 +722,7 @@ Framework routing is validated the same way, on a canonical app per framework: E
 - **You're on an old (pre-0.9) install.** Reinstall to get the bundled runtime — `curl -fsSL https://raw.githubusercontent.com/colbymchenry/codegraph/main/install.sh | sh` (macOS/Linux), `irm https://raw.githubusercontent.com/colbymchenry/codegraph/main/install.ps1 | iex` (Windows), or `npm i -g @colbymchenry/codegraph@latest`.
 - **`codegraph status` shows `Journal:` other than `wal`** — WAL couldn't be enabled on this filesystem (common on network shares and WSL2 `/mnt`), so reads can block on writes. Move the project (with its `.codegraph/` folder) onto a local disk.
 
-**MCP server not connecting** — Ensure the project is initialized/indexed, verify the path in your MCP config, and check that `codegraph serve --mcp` works from the command line.
+**MCP server not connecting** — Your agent starts the server itself, so you don't launch it by hand. Make sure the project is initialized and indexed (`codegraph status`) and that the path in your MCP config is correct. If it still won't connect, re-run `codegraph install` to rewrite the config.
 
 **Missing symbols** — The MCP server auto-syncs on save (wait a couple seconds). Run `codegraph sync` manually if needed. Check that the file's language is supported and isn't inside a `.gitignore`d or default-excluded directory (e.g. `node_modules`, `dist`).