refactor(cli): centralize TUI actions and key bindings - #1577

Summary

• Centralize CLI actions behind stable IDs and project Slash, Palette, Help, footer, and key bindings from one registry.
• Resolve optional user key overrides with legacy-default normalization, deterministic conflicts, and reserved recovery keys.
• Route Startup and Chat through state-aware central dispatch, including consistent mouse and popup navigation.

Scope

This is CLI product-line groundwork. It does not add HarmonyOS TUI/GUI support, change the renderer, or introduce platform-specific runtime code.

Compatibility and behavior

• Missing shortcut config keeps the current built-in behavior and is not persisted as a user choice.
• Prior generated default values are normalized; explicit non-default values are preserved.
• Ctrl+C and popup recovery bindings remain available; invalid or conflicting overrides are explained in Help.
• Idle and processing availability stays consistent across Slash, Palette, Help, footer, and dispatch.

Adversarial review fixes

• Normalize shortcuts only when the complete legacy-generated tuple matches; preserve mixed or partial explicit choices.
• Remove the unused single-variant action-source abstraction and make Startup handler classification exhaustive.
• Align popup recovery help with runtime behavior: Ctrl+W / Esc now maps to Close All Popups / Back.
• Keep real PTY/ConPTY coverage outside this PR; current coverage is crossterm event-level tests plus three-platform CI.

Validation

• cargo +nightly test -p bitfun-cli -- --nocapture (163 + 8 + 4 + 4 passed)
• cargo +nightly clippy -p bitfun-cli --all-targets (passed; existing repository warnings only)
• node scripts/check-core-boundaries.mjs
• pnpm run check:repo-hygiene
• git diff --check
• Two independent architecture and product adversarial review rounds; no remaining P0-P3 findings.