feat(client): leader s/w navigation pickers + scored, grouped command palette

[phall1]

What

Under the C-a leader:

• s opens a picker over all sessions (session-picker); choosing one re-attaches this client in-process via switch-session. A trailing "+ New session" row creates one.
• w opens a hierarchical window picker grouped by session: each session is a non-selectable header with its windows nested beneath. Selecting a window in the current session switches to it (select-window); selecting another session switches to it (switch-session) — its windows then list under its own picker.
• a stays a session-picker alias for muscle memory. No binding is left silently dead.

Command palette

• Scored fuzzy ranking. Matches are ranked best-first using contiguous-run, word-boundary/prefix, and earliness bonuses, so typing sp floats split-pane to the top.
• Category grouping. With an empty query, rows group under dim section headers — Pane / Window / Session / View. Headers fall away once a query narrows the list (a filtered view is a flat best-first ranking). Navigation and Enter skip header rows.
• Inline chord hints are preserved.

Bug fix

• Fix a usize underflow in the contiguous-run bonus when a match lands at index 0.

Docs

Made docs/consumers/tui.md and docs/CONFIG.md accurate to the shipped code:

• Corrected the prefix to C-a (was the stale ctrl+space) everywhere, including the defaults table and first-time-use walkthrough.
• Realigned the keybinding and action catalogs with the actual dispatcher (ACTION_NAMES) and the shipped default.toml bindings.
• Fixed the status-bar widget kinds to the ones actually implemented (session-name, time, windows) — the docs previously advertised nonexistent session / clock kinds.
• Removed the nonexistent phux config reload verb (the shipped subcommands are init / path / show); edits apply on client restart.
• Re-scoped the hooks section as schema-only / design-intent (no hooks ship in default.toml).

Tests

• select_list: scored-match (sp -> split-pane first), empty-needle, non-subsequence, word-boundary, stable-tie ranking; header drop-out under filtering; navigation/Enter skipping headers.
• action_registry: category headers in order, every row indented under a header, every registry action carries an in-order category.
• input_dispatch: window picker groups windows under their session, foreign sessions render a switch-session row.
• keybind: shipped-default resolver test for C-a s / C-a w / C-a a.

Local CI

just ci is green (fmt + lint + 1553 tests + deny + docs-check).

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