Persist provider-neutral session mappings and context checkpoints

## Parent and milestone

- Parent issue: #23
- Milestone: not scheduled; implementation starts after #15.

## Problem

Long work currently depends too heavily on one provider session. Provider history can grow slow, expire, or become unavailable, and cannot be treated as Durex canonical state.

## Outcome

Persist a portable, bounded checkpoint containing goals, decisions, repository state, validation, artifacts, completed work, and remaining work, with opaque mappings to provider sessions.

## Scope

- Define durable Durex session and checkpoint records.
- Map zero or more opaque provider sessions to one Durex work session.
- Create checkpoints on bounded lifecycle triggers and before runtime handoff.
- Support fresh-session and cross-runtime continuation.

## Non-goals

- Copying hidden reasoning or entire provider histories.
- Treating generated summaries as authoritative without artifact references.
- Multi-host transfer, which remains coordinated with #21.

## Acceptance criteria

- [ ] A task can resume from a checkpoint without the original provider session.
- [ ] Checkpoints contain provenance, size bounds, schema version, and artifact references.
- [ ] Secrets and raw terminal output follow documented retention and redaction rules.
- [ ] Targeted and conformance tests pass.
- [ ] Architecture, adapter, and operator documentation is updated.

## Validation

- Test checkpoint creation, schema migration, truncation, stale provider mappings, and cross-runtime handoff fixtures.

## Dependencies

#26 and the durable interaction work in #13.

Start by reading parent issue #23 and dependencies #26 and #13 to understand the existing durable interaction model and implementation boundary. Define the session and checkpoint design against the listed scope, then validate checkpoint creation, migration, truncation, stale mappings, and cross-runtime handoff. Done means the acceptance criteria pass, including bounded provenance, artifact references, retention rules, conformance tests, and updated documentation.

Written by the indexing model from the issue text.