[P0][AGENT-02] Common AgentSession adapter interface

Programme: #1
Epic: #20

## Objective
Create one internal protocol for local and external coding/agent providers while preserving provider-specific capability differences.

## Scope
- Session capability negotiation and version pinning.
- Start/resume/input/event/progress/artifact/approval/interruption/completion/diagnostics primitives.
- Structured error and termination reasons.
- Provider/model/inference-location disclosure.
- Artifact publication into the shared workspace store.
- Approval requests translated into trusted runtime proposals—not provider-owned confirmations.
- Golden protocol recordings and compatibility fixtures per provider/version.

## Trust rules
- Adapter integration is not proof the provider's internal file/shell/network tools are mediated.
- Session IDs are not durable task state.
- Prefer machine-readable provider protocols; interactive terminal parsing is not an acceptable primary embedding contract.

## Acceptance criteria
- [ ] Every required provider can report its actual capabilities through the common interface.
- [ ] Missing optional capabilities remain missing instead of being simulated misleadingly.
- [ ] Start/progress/artifacts/cancel/complete normalize into stable runtime events.
- [ ] Provider version/schema mismatch fails early with actionable diagnostics.
- [ ] Golden fixtures detect protocol drift.
- [ ] Provider session termination does not erase shared task/artifact state.

## Dependencies
- CORE-01
- SEC-04

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

## Task issues
- [ ] #449 `AGENT-02.T01` — Define the common lifecycle and optional capabilities
- [ ] #450 `AGENT-02.T02` — Implement supervised transport adapters
- [ ] #451 `AGENT-02.T03` — Normalize progress, artifacts and termination
- [ ] #452 `AGENT-02.T04` — Implement permission translation and trust classification
- [ ] #453 `AGENT-02.T05` — Build session context and destination policy
- [ ] #454 `AGENT-02.T06` — Implement interruption and resume semantics
- [ ] #455 `AGENT-02.T07` — Create golden protocol and shared workflow fixtures

Start by reading the stated dependencies CORE-01 and SEC-04, then inspect task issues #449–#455 to understand how the interface is decomposed. Use the acceptance criteria to define completion: normalized lifecycle events, honest capability reporting, early mismatch diagnostics, and provider-specific golden fixtures without losing shared task state.

Written by the indexing model from the issue text.