Migrate Codex execution behind AgentRuntime

## Parent and milestone

- Parent issue: #23
- Milestone: not scheduled; implementation starts after #15 and #42.

## Problem

Codex execution remains embedded in queue and Telegram call paths, while the
ACP and native Codex interfaces are not bound to Durex lifecycle, workspace,
cancellation, output, and session ownership.

## Outcome

Implement the first conforming `AgentRuntimeAdapter` using the pinned Codex ACP
path where sufficient, with approved native extensions and a documented PTY
fallback only for audited gaps.

## Scope

- Move Codex-specific command, session, ACP, and event mapping behind the
internal runtime contract.
- Bind the agent process to a fenced execution environment from #44.
- Preserve supervisor ownership and process-group cancellation.
- Use structured ACP/native events where supported and label PTY output as
degraded.
- Emit normalized and bounded provider-extension events.

## Non-goals

- Adding private queue or workspace methods to ACP.
- Changing Telegram UX.
- Removing compatibility paths before migration validation.
- Exposing Codex or ACP endpoints directly to remote clients.

## Acceptance criteria

- [ ] Existing Codex tasks behave identically through the adapter.
- [ ] ACP coverage and every native/PTY exception match #25 evidence.
- [ ] Resume, usage-limit wait, interactions, live output, cancellation, and
stale recovery remain covered.
- [ ] High-risk approval never relies only on degraded PTY text.
- [ ] The adapter passes runtime and execution-environment conformance suites.
- [ ] Targeted tests and operator documentation are updated.

## Validation

- Run full Durex tests plus ACP, approved native extension, and PTY fallback
integration tests against pinned versions.

## Dependencies

- #25, #26, #27, #44, #45, and #15.

Start with the internal AgentRuntimeAdapter contract and dependencies #25, #26, #27, #44, #45, and #15, then review their ACP, native-extension, execution-environment, and evidence requirements. Run the full Durex tests and the pinned ACP, native-extension, and PTY fallback integration tests. Done means conformance suites pass and targeted tests and operator documentation cover the listed acceptance criteria.

Written by the indexing model from the issue text.