Tasks: Documentation for pause/resume lifecycle

**Repo**: `coder/coder`

**Scope**: Create comprehensive documentation for task pause/resume lifecycle feature.

**Changes**:
- **AI agent compatibility matrix**: Document which AI agents support session persistence, what context is lost without it, requirements for full support
- **Persistent storage requirements guide**: Document why persistent storage is needed, required paths (AgentAPI state file, AI agent session dirs like `.claude/`), minimum storage recommendations, troubleshooting missing storage
- **Task lifecycle user guide**: Document how pause/resume works (automatic pause, manual pause, resume), what gets preserved, viewing logs while paused
- **Registry module README updates**: Document how the AgentAPI module handles shutdown scripts, state file paths, and the new variables (`state_file_path`, `pid_file_path`, `enable_state_persistence`)
- **Provider shutdown timeout configuration**: Document how different providers may have different timeouts for allowing workspace agents to gracefully shut down, and how this will affect task snapshotting. Consider mentioning infra-based timeouts as well (e.g. Kubernetes)

**Files** (example):
- `docs/ai/compatibility.md` (new)
- `docs/ai/persistent-storage.md` (new)
- `docs/user-guides/task-lifecycle.md` (new)
- `registry/coder/modules/agentapi/README.md` (update)

**Acceptance criteria**:
- [ ] Compatibility matrix lists all AI agents with support level (full/partial/none)
- [ ] Storage guide explains persistent volume requirements with provider-specific examples
- [ ] User guide covers all pause/resume workflows with CLI and UI examples
- [ ] Registry README documents new variables and shutdown script behavior
- [ ] Integrate provider shutdown timeout configuration from [#6174](https://github.com/coder/coder/issues/6174) and consider documenting how infra-based timeouts may also play a role (e.g. Kubernetes)
- [ ] All docs reviewed and approved before GA release

**Dependencies**:
- All Beta and GA issues complete

**References**:
- [RFC: Tasks: Start/Pause/Resume Lifecycle](https://www.notion.so/coderhq/Tasks-Start-Pause-Resume-Lifecycle-2cdd579be592805a929fc36d0c82d65d): Documentation section
- https://github.com/coder/coder/issues/6174