feat(core): warn on concurrent active checkout sessions

[shiny-code-bot]

Summary

• Add a small active-session presence registry under CODE_HOME/state/active-sessions for write-capable Every Code sessions.
• Warn after session startup when another live write-capable session is already active in the same canonical checkout, while allowing separate git worktrees to stay quiet.
• Thread SessionSource into core session startup so TUI and code exec warnings identify the correct surface.
• Factor PID liveness out of housekeeping so active-session cleanup and worktree cleanup share one implementation.

Fixes #231.

Validation

• cargo test -p code-core active_sessions --lib
• cargo test -p code-core exec_session_warns_when_checkout_already_has_write_capable_session --test active_session_warnings
• git diff --check
• ./build-fast.sh