[P0][AGENT-03] Codex integration

Programme: #1
Epic: #20

## Objective
Integrate OpenAI Codex through its documented structured application/server surface behind `AgentSession`, with containment, artifacts and independent verification owned by our runtime.

## Scope
- Pin supported Codex/App Server versions and generated/matched schemas.
- Start/resume/cancel session lifecycle through structured protocol.
- Translate Codex progress, tool/approval events, artifacts and errors into AgentSession events.
- Stage only the approved repository/workspace into the provider environment.
- Map trusted runtime approvals rather than delegating authorization to provider UI.
- Persist provider session/thread IDs only as secondary references.
- Collect patches/artifacts into CODE-01 review flow.

## Security / product rules
- Codex does not receive the user's home directory, browser profile, credential vault or unrelated personal graph.
- Provider completion never bypasses independent tests/verification.
- Model/provider execution location and data destination are visible.

## Acceptance criteria
- [ ] A bounded repository fixture starts, makes a scoped change, publishes artifacts and terminates cleanly.
- [ ] Cancellation stops the provider process/session and prevents later dispatch under revoked leases.
- [ ] Independent tests determine verified completion.
- [ ] Unsupported/changed protocol versions fail honestly with diagnostics.
- [ ] No unapproved workspace files/secrets appear in provider context.
- [ ] Crash/restart preserves runtime task/artifact state even if provider session cannot resume.

## Dependencies
- AGENT-02

**First phase:** P0
**Maturity target:** P3
**Owner:** harness-pc-providers

## Task issues
- [ ] #456 `AGENT-03.T01` — Pin and probe the supported Codex build
- [ ] #457 `AGENT-03.T02` — Implement App Server session lifecycle
- [ ] #458 `AGENT-03.T03` — Translate approvals and tool events
- [ ] #459 `AGENT-03.T04` — Stage context and collect artifacts
- [ ] #460 `AGENT-03.T05` — Implement interrupt, crash and resume handling
- [ ] #461 `AGENT-03.T06` — Qualify the shared coding workflow

Start by reading the AgentSession contract and dependency AGENT-02, then use task issues #456–#461 to divide the integration work. Validate against the bounded repository fixture and acceptance criteria: scoped artifacts, cancellation, independent verification, honest protocol diagnostics, workspace containment, and crash/restart state preservation.

Written by the indexing model from the issue text.