Add Claude context usage hint via hooks in status bar

[0dragosh]

Motivation

• Surface Claude/Context usage to the user as a bottom legend/hint so TUI sessions in zellij/tmux can show how much conversation context is used and where /context hooks can be leveraged.
• Accept and persist context usage reported by Claude hook payloads (or parsed from notification messages) per worktree so the UI can display it immediately and across refreshes.

Description

• Add context_usage_percent: Option<u8> to the Worktree model to persist hook-derived context percentages (file: src/worktree/model.rs).
• Extend HookEvent::SessionNotification to accept an optional context_usage_percent field and add context_usage_percent() plus parse_context_percent_from_message to extract a percent either from the explicit field or from message text (file: src/hooks/event.rs).
• Update generated Claude hook scripts in src/hooks/install.rs to forward message and a best-effort context_usage_percent value (explicit payload field or % parsed from message) over the cwt Unix socket.
• Wire hook handling in App::handle_hook_event to update and persist context_usage_percent on session notification events (file: src/app.rs).
• Extend the status bar API and rendering to accept an optional context percentage and show a compact hint Claude ctx:<n>% (/context via hooks) in the bottom legend, including tests for rendering (file: src/ui/status_bar.rs).

Testing

• Ran cargo check and it completed successfully.
• Ran cargo clippy --all-targets -- -D warnings with no warnings (success).
• Ran cargo test and all tests passed (unit + integration), including the new parsing and status bar rendering tests.

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