Support multiple PRs/sessions per work item

[dpuglielli]

Problem

The current plugin architecture assumes 1 issue = 1 branch = 1 session. This breaks down for:

• Large issues that need multiple PRs
• Iterative development with incremental PRs
• Team handoffs where multiple people work on the same issue
• Rework/revisions after initial PR is merged

Solution

Enable 1 issue = N sessions with two relationship types:

• Phases - Sequential work with dependencies (Phase 2 depends on Phase 1)
• Slices - Parallel work within a phase (can be done in any order)

Design Summary

Aspect	Decision
Relationship model	Phases (sequential) + Slices (parallel)
Session discovery	Auto-detect existing sessions, prompt for relationship
Dependencies	Tracked in session file `Depends-on` field
Branching	Rebase from main after phase merges
Branch naming	`issue/--p/`
Acceptance criteria	Issue-level, progressively checked
Issue closure	Explicit `/onus:close` with validation
Data storage	Session files only (no central index)
Backward compat	Existing sessions = phase 1, single phase

Acceptance Criteria

Phase 1: Data Model

• Add `parsePhaseFromBranch()` to shared module
• Add `findSessionsForIssue()` to shared module
• Update session templates with Issue, Phase, Slice, Depends-on fields
• Rebuild shared bundles

Phase 2: Memento Workflow

• Add session discovery to session-startup.js
• Update /memento:start command spec for multi-session
• Add phased branch name generation
• Add next phase/slice calculator

Phase 3: Onus Integration

• Update /onus:status for multi-session aggregation view
• Update /onus:close with session validation

Phase 4: Documentation

• Update sessions.md context file
• Update work-items.md context file
• Integration testing

References

• Design doc: `docs/plans/2026-01-01-multi-session-work-items-design.md`
• Implementation plan: `docs/plans/2026-01-01-multi-session-implementation.md`