feat(stockbit): one browser sign-in in idx-core for the CLI and MCP

`stockbit::signin` is now the single sign-in flow. `idx stockbit login`
waits on it in the foreground; the `stockbit_login` MCP tool runs it on a
thread and `stockbit_status` reports `sign_in.state`, so an agent knows when
to stop polling. Auto uses a browser unless the shell is over SSH or has no
display, and falls back to the local page only when the browser cannot start.

The browser way opens Stockbit's sign-in page in a profile only idx uses and
reads the session cookie over DevTools. What a real sign-in showed:

- The cookie is host-only on stockbit.com, so deleting by the assumed
  .stockbit.com removed nothing and left live tokens in the profile. It is
  now deleted by its own domain and path, with every other tab closed first,
  and read back to confirm.
- A signed-out page writes the cookie without tokens; it no longer counts.
- A profile belongs to one browser, so it records the executable that made it.
- On macOS the config directory depends on XDG_CONFIG_HOME, so a shell and an
  app started from the Dock disagreed; both default locations are now checked.
- A thread does not outlive main, so idx-mcp closes a waiting browser when
  stdin closes or on SIGTERM or Ctrl-C.

Progress goes to stderr, which keeps `-o json` to one document on stdout.

Google refused its own sign-in in this window on 2026-10-31, most likely over
the debugging flag; password sign-in works, and the docs and roadmap say so.

Claude-Session: https://claude.ai/code/session_01RqPPVaoFxLToGAXXBEoCtH

crates/idx-mcp/Cargo.toml

@@ -23,7 +23,7 @@ rmcp = { version = "3.5", default-features = false, features = ["server", "macro
 schemars = "1"
 serde.workspace = true
 serde_json.workspace = true
-tokio = { version = "1", features = ["macros", "rt-multi-thread", "io-std", "sync"] }
+tokio = { version = "1", features = ["macros", "rt-multi-thread", "io-std", "signal", "sync"] }
 
 [dev-dependencies]
 # Tests drive the binary with the mock provider; release builds exclude it.