[codex] Align Fleet watch navigation and collaboration

[jiunbae]

What changed

• align multi-host Fleet j/k navigation with native watch sibling semantics, including singleton-parent fallback and wrapping
• retain J/K as explicit global agent-pane jumps
• reuse the native watch theme throughout Fleet instead of Fleet-only hard-coded colors
• add prefix+S as the Fleet popup while preserving prefix+s for local watch
• make a/A, m/M, message skills, kind/mode persistence, mailbox claim, and reply available in the multi-host Fleet hierarchy
• route durable collaboration to the selected node over the existing exact-pane SSH relay with capability and access-mode gates
• update protocol, install, Fleet, README, and changelog documentation

Why

The multi-host Fleet hierarchy had drifted from the established muxa watch interaction model. Navigation behaved differently, the visual palette was inconsistent, and entering Fleet hid Ask and durable collaboration actions that users already rely on in local watch.

Impact

Operators can move between host/session/window/pane nodes using the same mental model as native watch, open Fleet directly with prefix+S, and continue using Ask, messages, skills, and mailboxes without descending to a pane manually. Older remote nodes fail Fleet collaboration with a clear upgrade instruction rather than receiving unsupported relay frames.

Self-review

• isolated this PR from an unrelated Work/Dashboard commit that preceded the original development branch
• verified exact-pane identity checks, observe/control mutation gates, mixed-version capability negotiation, bounded mailbox frames, and node-local durable storage
• corrected one stale documentation reference from lowercase j/k pane jumps to uppercase J/K
• found no remaining merge-blocking issues

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo build --workspace --bins
• cargo test --workspace (709 core tests passed, 1 environment-gated rmux test ignored; 700 CLI tests, 12 e2e tests, 70 muxad tests, and all remaining workspace/doc tests passed)
• local install verified with live Fleet collaboration capability and active tmux prefix+S binding