[IR-P6][WS5] Add Session/UnitOfWork abstraction

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Problem statement

Phase 6 deliverable 1 needs a dedicated implementation issue to ensure scope clarity and testable acceptance criteria.

Intended invariant

Implement session/unit-of-work abstraction covering identity map and transaction state.

Scope boundaries

• Implements only this Phase 6 deliverable
• Coordinates with sibling Phase 6 issues through the epic
• No out-of-scope feature expansion

Deliverable checklist

• Implementation complete for this deliverable
• Tests added/updated and passing
• Migration impact recorded with rationale
• Docs impact recorded with required updates landed
• Evidence linked for roadmap phase gate

Validation plan

Define and execute deterministic tests for this deliverable; attach CI/verification evidence before closing.

Rollback/failure mode

If acceptance criteria cannot be met in this scope, split into follow-up issue(s) and update roadmap + epic links immediately.

Parent epic

#96

Roadmap reference

Roadmap reference: docs/plans/2026-06-19-001-ir-first-roadmap.md (Phase 6)

Migration impact

minor - expected impact for this deliverable (adjust if needed with explicit rationale).

Documentation impact

update-existing - evaluate guide/API/concepts/examples and update as required.

Sync status

Synced with roadmap: 2026-06-19

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Refresh sync update: reviewed against current roadmap and Phase 7/8 sequencing (v0.13.0 cutover). No scope change required for this issue at this time.

Roadmap reference: docs/plans/2026-06-19-001-ir-first-roadmap.md.