docs: Phase 7 - Controlled Write Parallelism design document

[roomote-v0]

Related GitHub Issue

Addresses: #12330

Description

This PR attempts to address Issue #12330 by formalizing Phase 7 (Controlled Write Parallelism) as proposed by @easonLiangWorldedtech in the issue discussion. Feedback and guidance are welcome.

What changed:

• Created docs/architecture/phase-7-controlled-write-parallelism.md -- a full architectural design document covering Phase 7, which absorbs the deferred Phase 3d scope and extends it with supporting infrastructure
• Updated docs/architecture/phase-6-background-task-visibility.md to add forward references to Phase 7 for deferred items (write-capable background tasks and persistent history)

Phase 7 is organized into three sub-phases:

1. Phase 7a: File Lock Manager -- advisory file-level locking so concurrent tasks coordinate write access safely
2. Phase 7b: Write-Capable Background Tasks -- removes the read-only restriction from background tasks behind an explicit allowWrites parameter, with conflict detection
3. Phase 7c: Persistent Background Task History -- background task metadata and conversation history survive VS Code restarts

The design document includes risk analysis, testing strategy, dependency chain, open questions, and an updated roadmap table.

Branch stack:

main
  +-- feature/enhanced-subtask-handoff-summary (Phase 1)
        +-- feature/phase-2-3-combined (Phase 2+3)
              +-- feature/remaining-phases-4-5-6 (Phase 4+5+6)
                    +-- feature/phase-7-write-parallelism (Phase 7 - this PR)

Test Procedure

This is a documentation-only change (two .md files). No code changes, no tests affected.

• Verified the design document is consistent with the existing Phase 6 design doc and current codebase architecture
• Verified forward references in the Phase 6 doc point to the correct sections

Pre-Submission Checklist

• Issue Linked: This PR is linked to an approved GitHub Issue (see "Related GitHub Issue" above).
• Scope: My changes are focused on the linked issue (one major feature/fix per PR).
• Self-Review: I have performed a thorough self-review of my code.
• Testing: New and/or updated tests have been added to cover my changes (if applicable).
• Documentation Impact: I have considered if my changes require documentation updates (see "Documentation Updates" section below).
• Contribution Guidelines: I have read and agree to the Contributor Guidelines.

Documentation Updates

• Yes, documentation updates are required. This PR IS the documentation update -- it adds the Phase 7 design document.

Additional Notes

This PR is a design document only. Implementation of Phase 7a/7b/7c will follow in subsequent PRs once the scope is confirmed through discussion on Issue #12330.

Interactively review PR in Roo Code Cloud

[roomote-v0]

Closing this draft PR as the work has been consolidated into #12356.