Add cogitate coder mode: write flag, handoff command, coder agent
Three composable changes:
1. `write: true` frontmatter flag — any cogitate muse agent can opt
into full shell/write access. Each provider interprets the flag:
Anthropic drops --allowedTools restriction, OpenAI switches sandbox
to write mode, Google drops --allowed-tools prefix constraint.
2. `sol call handoff <agent>` — fire-and-forget agent spawning from
stdin. Reads prompt, calls cortex_request(), prints agent_id, exits.
Same ergonomics as hub request create and hop implement.
3. `muse/coder.md` — first write-enabled cogitate agent. Carries
development guidelines inline (from body skill content). Receives
implementation tasks via handoff, executes with full repo access.
15 new tests covering write flag per-provider, handoff success/error,
and coder agent frontmatter validation.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>