docs(openspec): propose customizable keymap UI

[criscool]

Summary / 摘要

OpenSpec proposal for a user-overridable keybinding API on EditorAPI, unblocking Roadmap #14 (Custom keymap UI). Proposal only — no implementation code.

Motivation / 背景与动机

• Roadmap: docs/ROADMAP.md docs(openspec): propose realtime collaboration plugin #14 (Custom keymap UI, P2) — explicitly blocked on "expose keymap register / query API first"

Nexus currently has three unrelated shortcut surfaces (NexusPlugin.shortcuts, EditorCommand.hotkey, plugin cmExtensions direct injection) with no discovery / override / persistence contract. Following CONTRIBUTING.md §3.1: new public capability → OpenSpec proposal required before implementation.

Companion implementation PR will be opened right after this one (branch feat/core-editor-apis), so reviewers can see the proposal and its execution together.

Changes / 变更内容

• openspec/changes/add-customizable-keymap-ui/:
  • proposal.md — Why / What / Impact
  • tasks.md — 5-phase implementation checklist
  • design.md — Context / Goals / 4 key decisions with alternatives / Risks / Migration / Open Questions
  • specs/editor-core/spec.md — ADDED Requirements: User-overridable keybindings with 9 scenarios covering discovery, rebinding, unbinding, restore, conflicts, unknown-id tolerance, initial hydration

Testing / 测试

N/A — proposal-only PR. Implementation coverage lives in the companion PR.

Compliance / 合规自检

• CLA signed (first-time contributor via CLA bot)
• AI disclosure: proposal wording refined with AI assistance; the technical decisions (two-layer override model, id format, conflict surfacing policy, non-goals) are mine.
• No new dependencies
• No build artifacts / secrets committed

Checklist / 自检清单

• Title follows Conventional Commits (docs(openspec): ...)
• Follows OpenSpec three-stage workflow (Stage 1: create proposal)

Precedent for proposal-only PR: #15 docs(openspec): propose cloud storage interface.

[CLAassistant]

All committers have signed the CLA.

[redreamality]

Reviewed PR #117.

This is a documentation-only OpenSpec proposal for customizable keymap UI. The scope is appropriately narrow and matches the expected proposal structure:

• proposal.md explains why customizable keybindings are needed.
• design.md covers registry / override-layer / persistence / conflict-detection considerations.
• specs/editor-core/spec.md defines expected editor-core behavior.
• tasks.md breaks the implementation into reviewable phases.

Validation notes:

• CLA is passing.
• The PR only adds OpenSpec documentation and does not touch runtime code.
• I also reviewed the related implementation PR #118; this proposal is useful as a standalone design artifact even if implementation details need to be reviewed separately.

Conclusion: approved from OpenSpec/proposal review perspective.