feat: align generation concurrency, frame prompts, and MCP skills

[erickkkyt]

Summary

• Delegate account-level generation concurrency to the active Provider by removing BeatDesign's cross-Project mutex and per-Project running-task limit.
• Move first/last-frame intent entirely into canonical @ImageN prompt directives, keep image references generic, and advance the asset-first request contract to v2.
• Add the versioned built-in Skill catalog foundation through MCP Resources plus bdesign_skill_list / bdesign_skill_get, including WorkBuddy runtime packaging and probes.
• Synchronize architecture, provider, MCP, release-scope, PRD, README, and changelog documentation.

Behavioral boundaries

• The short local submission lock remains only for intent/history consistency; Provider requests execute outside that lock.
• Attachment order alone does not assign first or last frames.
• Only canonical prompt syntax such as Use @Image1 as the first frame. is translated into Provider frame fields; non-canonical wording remains a generic reference.
• Provider-specific hard media constraints remain enforced.
• The official bundled Skill catalog is intentionally empty until a creative Skill passes prompt evaluation and visible workflow verification.

Verification

• pnpm typecheck
• pnpm test — 395 passed, 0 failed
• pnpm i18n:check — 615 aligned keys for English, Chinese, and Japanese
• pnpm build
• MCP stdio and HTTP contract tests
• WorkBuddy Connector structure tests
• Diff whitespace and secret-pattern checks

Verification boundary

No real paid BeatAPI request was submitted. Provider-controlled parallel generation across two live Projects remains a separate credentialed end-to-end check.

Review

Pre-landing review found no blocking data-safety, concurrency, contract, or documentation issues. No frontend visual changes are included.

Documentation

• README.md / README.zh-CN.md / README.ja.md: aligned the 29-tool MCP catalog and built-in Skill discovery wording.
• ARCHITECTURE.md / PROVIDERS.md: documented Provider-owned concurrency and prompt-only frame intent.
• MCP.md / PRODUCT_PLAN_AND_STATUS.md / BEATDESIGN_V0_2.md: documented request contract v2, generic references, Skill Resources, and the empty official catalog boundary.
• RELEASE_SCOPE.md / CHANGELOG.md: synchronized the v0.2.3 candidate gate and Unreleased summary.