feat: jump to session via terminal adapter registry (cmux/tmux/iTerm2)

[Valgard]

What

Enter "jump to session" worked only inside tmux. This refactors the inline tmux logic into a TerminalJumper registry under src/jump/ (one adapter per file) and adds two backends:

• cmux — read CMUX_WORKSPACE_ID from the agent process env, then cmux select-workspace --workspace <uuid>.
• iTerm2 — match the process's controlling tty to an iTerm2 session and focus it via AppleScript (select pane/tab/window + activate).
• tmux — existing behavior preserved (now an adapter; "PID not in any pane" became NotApplicable so other backends can try).

How

resolve() walks jumpers() in order (cmux → tmux → iTerm2). The three-way JumpAttempt (NotApplicable / Jumped / Failed) makes adapters composable — first applicable wins, NotApplicable falls through. Backends are mutually exclusive by controlling tty.

Tests / verification

• 15 new unit tests for the registry loop and the parsing helpers.
• Live-tested on macOS / iTerm2 — focus lands on the correct pane/tab/window.
• Full CI gate green locally on rustc 1.96.0: cargo clippy -- -D warnings, cargo test (180 passed), cargo build --release.

Notes

• iTerm2's first jump triggers a one-time macOS Automation permission prompt; until granted it surfaces as a status message, not a crash.
• Docs updated in AGENTS.md (Key Bindings + new "Terminal Jump" section).

Closes #137