feat: filter sessions by state with Tab/Shift+Tab in menu

[kbwo]

Closes #310.

Problem

The menu could only filter sessions by text (branch, path, session name). There was no way to narrow the list to the sessions that need attention (busy/waiting) versus those free for new work (idle).

Approach

Issue #310 proposed two UX options (a s-then-letter sub-mode, or a cycling toggle). This implements a refined version of the cycling option: a session-state filter as an independent dimension that composes with the existing text search, driven by a single keypress.

• Tab cycles the filter forward: All → Busy → Waiting → Idle → All; Shift+Tab cycles backward.
• The active filter is shown in the footer, alongside a Tab-State Filter hint in the controls line.
• The state filter and the text search are applied in one pipeline, so both can be active at once.

Why this over the issue's options: a single keypress is fastest (the core need), it adds no separate keymap/mode to learn, and Tab/Shift+Tab don't collide with the existing top-level shortcuts.

Details

• The four internal session states collapse into three user-facing categories: pending_auto_approval folds into Waiting, mirroring the status display in statusIcons.ts.
• Rows without a running session have no state, so they are excluded under any specific filter (shown only when the filter is All).
• Filter logic lives in filterByQuery.ts next to the existing text filter and is unit-tested.

Known limitation

Tab does not cycle while the text-search input is active (/ mode), matching the existing handler that early-returns in search mode. The query is preserved when leaving search mode, so both filters can still be combined; simultaneous "type and Tab" would need follow-up.

Verification

• npm run typecheck, npm run lint pass.
• New unit tests in src/utils/filterByQuery.test.ts (filter categories, pending→waiting folding, session-less exclusion, forward/backward cycling, labels) pass; full suite: 1760 passed. The 6 failing files are pre-existing *.submodule.test cases that fail in this sandbox because git config --global cannot write ~/.gitconfig — unrelated to this change.

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