feat(mcp): session-isolated context documents to prevent multi-session overwrites

[JeremyDev87]

Purpose

The current single docs/codingbuddy/context.md file is overwritten when multiple sessions run simultaneously (e.g., taskMaestro conductor + workers, or multiple Claude Code sessions). PLAN mode resets the entire file, destroying other sessions' context.

Problem

Session A (conductor): PLAN → resets context.md
Session B (worker 1):  ACT → appends to context.md
Session C (worker 2):  ACT → appends to context.md
  ↓
Session A resets → B and C's context lost
B and C append simultaneously → race condition

Observed in 2026-03-21 parallel execution: worker sessions overwrote conductor's context.

Solution: Session-Isolated Context Files

docs/codingbuddy/
├── context.md                        ← Index (active session list)
├── sessions/
│   ├── ctx-{session-id-A}.md         ← Conductor session
│   ├── ctx-{session-id-B}.md         ← Worker 1 session
│   └── ctx-{session-id-C}.md         ← Worker 2 session
└── shared/
    └── decisions.md                  ← Cross-session shared decisions (append-only)

Changes

• apps/mcp-server/src/ — Modify update_context and parse_mode context handling
• Add sessionId parameter to update_context
• PLAN reset only affects own session file, not others
• shared/decisions.md for cross-session communication (append-only, no reset)
• context.md becomes an index listing active sessions

Session ID Strategy

• Auto-generated UUID per session (stored in session state)
• Fallback: if no sessionId provided, use legacy single-file behavior (backward compatible)
• taskMaestro integration: session ID derived from pane-{N} for easy mapping

API Changes

// update_context — add sessionId
update_context({
  mode: "ACT",
  sessionId: "abc-123",         // NEW
  progress: [...],
  shareDecisions: ["..."]       // NEW: also written to shared/decisions.md
})

// parse_mode — returns session-specific path
parse_mode("PLAN: ...") → {
  contextFilePath: "docs/codingbuddy/sessions/ctx-abc-123.md",
  sharedDecisions: [...],        // from shared/decisions.md
}

Backward Compatibility

• No sessionId → falls back to single context.md (existing behavior)
• sessions/ directory auto-created on first session-isolated call
• Existing context.md content preserved

Acceptance Criteria

• update_context accepts sessionId parameter
• Each session writes to its own file (sessions/ctx-{id}.md)
• PLAN reset only affects own session file
• shared/decisions.md for cross-session append-only data
• context.md serves as index of active sessions
• No sessionId → legacy single-file behavior (backward compatible)
• Race condition free (each session writes to different file)
• TDD: tests first

References

• Current implementation: apps/mcp-server/src/mcp/ (context handling)
• context-management skill: packages/rules/.ai-rules/skills/context-management/
• Related: feat: add session issue tracking to update_context #842 (session issue tracking)
• Incident: 2026-03-21 parallel execution context overwrites