mecatui: live-reload operator settings (keymap) without restart

Problem

mecatui reads the operator-tier keymap: overrides from ~/.config/mecatl/settings.yaml exactly once at startup — applyKeyOverridesToDeps in cmd/mecatui/main.go runs before tea.NewProgram, bakes the result into ui.Deps.KeyOverrides, and ui.New applies it at construction (model.go). There is no re-read path: no fsnotify watch, no /reload palette command, no signal handler. An operator who edits settings.yaml must quit and relaunch mecatui to pick up a keybinding change.

Why it's worth fixing

Keybinding remapping is an iterative, taste-driven activity — an operator trying to find a comfortable home for Agents/Effort (e.g. to free ctrl+a/ctrl+e for line editing, see #455) will want to try a chord, feel it, and adjust. A full restart per iteration is friction that discourages the very configurability the feature offers.

Possible shapes (design TBD)

• A /reload palette command that re-reads settings.yaml, re-validates the keymap, and swaps m.keys in place (rebuilding help markings).
• An fsnotify watch on the settings file that does the same automatically on save.
• Either must handle: an invalid edited file (keep the last-good keymap + surface a loud warning, don't crash or drop bindings), and chords whose action is mid-overlay (e.g. remapping CancelChild while the agents overlay is open).

Scope note

The validator (keymap.Parse/keymap.Validate) and the override application (applyKeyOverrides) already exist and are pure — the work is the reload trigger + safe in-place swap + a warning surface, not new keymap machinery. See #455 for the documentation side of the keymap feature.

Read cmd/mecatui/main.go and model.go, starting at applyKeyOverridesToDeps and ui.New to trace how key overrides are loaded and applied. Define a reload trigger and safe in-place swap using the existing validator, preserving the last-good keymap and surfacing invalid-file warnings, including when an overlay is active.