feat(tui): add command palette with read-only agent management commands

[bug-ops]

Summary

• Introduce vim-style command palette (: key) for TUI dashboard with fuzzy-searchable overlay
• Phase 1 implements 7 read-only commands: skill:list, mcp:list, memory:stats, view:cost, view:tools, view:config, view:autonomy
• Dedicated mpsc channel for TUI → Agent communication; most commands resolve locally from MetricsSnapshot
• Command dispatch via closed enum — no user input evaluation

Changes

• New: crates/zeph-tui/src/command.rs — TuiCommand enum, CommandEntry, command_registry(), filter_commands()
• New: crates/zeph-tui/src/widgets/command_palette.rs — overlay widget with fuzzy search and keyboard navigation
• Modified: app.rs, channel.rs, event.rs, lib.rs, widgets/mod.rs, src/main.rs
• 888 lines added, 35+ new tests (1908 total pass)

Test plan

• cargo +nightly fmt --check passes
• cargo clippy --workspace -- -D warnings passes
• cargo nextest run --workspace --lib --bins — 1908 tests pass
• All 7 commands tested (registry, fuzzy filter, execution)
• Keyboard navigation tested (open/close, arrow keys, Enter, Escape)
• Security audit: no injection vectors, bounded channels, no secrets exposed

Closes #572