feat(tui): add configurable keybindings via config

[jongio]

Summary

Adds a keybindings config map so users can remap any keyboard shortcut in the TUI. Closes #210.

Keys in the map are action names (for example search, quit, cmd_palette) and values are comma-separated key lists (for example /,ctrl+f). Listed actions replace their default keys; any action you do not list keeps its default.

What changed

• internal/config/config.go: new Keybindings map[string]string field (keybindings in JSON, omitempty).
• internal/tui/keys.go: refactored the package key map so it is produced by defaultKeyMap(). Added a registry of remappable actions (keybindingEntries, keybindingActionNames), a parseKeyList helper, and a pure applyKeybindingOverrides that returns a new key map plus warnings.
• internal/tui/model.go: NewModel applies user overrides to the global key map right after theme resolution and logs any warnings via slog.
• README.md: documents the option, adds a "Customizing Keybindings" section with the full action-name list, and updates the example config.
• docs/keybindings.md: note at the top pointing to the remap capability.

Behavior and safety

• Unknown action names are ignored with a logged warning.
• An override that has no valid keys is ignored with a warning.
• If a remap collides with a key another action uses, that remap is dropped and the default is kept, so a bad entry never leaves an action unreachable. Conflict resolution is deterministic (sorted) and terminates because the built-in defaults never collide.
• Each remapped action keeps its original help description; the help hint text is regenerated from the new keys.

Acceptance criteria

• Users can remap shortcuts through config.json.
• Unlisted actions keep their defaults.
• Invalid or conflicting entries fall back to defaults instead of breaking input.
• The full list of action names is documented.

Testing

• go build ./...
• go vet ./...
• golangci-lint run ./... (0 issues)
• go test ./... -count=1 (all packages pass)

New internal/tui/keys_test.go covers no-op overrides, simple remaps, description preservation, unknown actions, empty values, default-conflict reverts, conflict-free swaps, two-override conflicts, key de-duplication, base immutability, and an end-to-end key.Matches check on a remapped binding.