docs: prepare develop for main release

[britle20]

Summary

• Refresh README to describe the current browser DAW prototype instead of the early scaffold plan.
• Update PLANS.md for develop-to-main stabilization and clear Active Issue Order.
• Mark completed feature specs as Done so PLANS.md and docs/features status values agree.
• Sync the local GitHub Wiki clone and review the latest user-edited wiki content.
• Update Wiki Home links to point directly to 1주차, 2주차, and 3주차 회고 pages.

Changes

• Removed the long feature-spec list from README and replaced it with a compact documentation map.
• Added current implemented capabilities, known limits, and workflow notes to README.
• Kept the product goal wording as music creation tool.
• Split PLANS.md into current stabilization focus, completed milestones, planned milestones, backlog, and frozen scope.
• Normalized completed feature spec statuses from Complete/In Review to Done.
• Manually closed Multiple local project management #59 because feat: add multi-project management #61 merged into develop and did not auto-close the issue.

Testing

• npm run typecheck
• npm run lint
• npm run test
• npm run build
• git diff --check

Screenshots

Not applicable; documentation-only change.

Notes / Limitations

• The Wiki Home update was committed directly to the GitHub Wiki repository because wiki changes do not go through this app repository PR.

Follow-up

• None for this release-prep docs pass.