Add a harness adapter

The adapter owns the spawn recipe, including WindowsShell: PowerShell, Cmd for a .cmd shim, or Direct. spawn_environment::wrap in src-tauri/src/agent/spawn_environment.rs reads that declaration. Do not hard-code a shell in the spawner. On macOS and Linux every provider uses Direct.

1. Add one file under src-tauri/src/agent/provider/adapters implementing AgentProvider. The trait is src-tauri/src/agent/provider/mod.rs. Register the module and its static in src-tauri/src/agent/provider/adapters/mod.rs.
2. Add the Provider variant and the adapter() arm in src-tauri/src/models/agent.rs. Add the id to BUILTIN_HARNESS_IDS in src-tauri/src/agent/provider/mod.rs. claude is an alias of anthropic via HARNESS_PROFILE_ALIASES in src-tauri/src/agent/harness_catalog.rs, not a second provider.
3. Add an inspector_label arm in src-tauri/src/agent/harness_catalog.rs. The match is exhaustive. capabilities() on the adapter is the source of truth. builtin_harness_catalog() writes the table. Do not hand-write a TypeScript capability map.
4. Declare circuit_observation() for the observation the harness really supports. The default is unwired. Do not add a harness-name branch under src-tauri/src/services/circuit_worker. The contract is docs/development/circuit-session-observation.md.
5. Cover a fresh spawn and a resume. When a test runs a real CLI, record that CLI's version next to the assertion.
6. Run cargo test from src-tauri/ and commit src/types/generated/HarnessCapabilitiesTable.ts and src/types/generated/HarnessCapabilitiesTable.json.
7. Update the harness row in docs/user-guide.md. Add a caveat to docs/troubleshooting.md when runtime behaviour needs one. Touch README.md only when the user-facing harness list changes. Put externally verified CLI notes in docs/learning with a source link, not in the user guide as an assumption.

From src-tauri/: cargo test --lib harness_catalog. Then, from the worktree root, npm run check:docs.