Document Bob support in supported agents and host-surface parity

## Context

Any new host integration must be reflected in user-facing docs and the host-surface parity matrix.

Parent epic: #890

## Scope

Add Bob support documentation once the implementation shape is selected.

## Tasks

- [ ] Update `docs/reference/supported-agents.md` with Bob support status and setup notes.
- [ ] Update `docs/host-surface-parity.md` with Bob row(s), including parity status and guidance style.
- [ ] Document Bob Shell auth and license gates: IBM login, `BOBSHELL_API_KEY`, `--show-license`, `--accept-license`.
- [ ] Document Bob command path and skill path.
- [ ] Document support limitations and verification status.
- [ ] Link to the Notion competitive research page from the issue or internal docs if appropriate.

## Acceptance Criteria

- [ ] Docs distinguish Bob IDE and Bob Shell behavior.
- [ ] Docs do not claim full support before smoke tests pass.
- [ ] Host-surface inventory tests pass after adding Bob row(s), if applicable.