Rewrite unified.md as identity-first agent, add chat_context pre-hook
Replace the instruction-heavy unified muse prompt with a ~140-line
identity-first version. Sol is now "the journal itself" rather than
a generic assistant. Inline CLI command docs replaced by a skill
trigger table. Runtime-specific instructions (location context,
system health, awareness conditionals, behavioral defaults) move
to a new chat_context pre-hook that conditionally injects them.
- muse/unified.md: new frontmatter (no system prompt, hook→chat_context),
identity narrative using template vars, skill trigger table
- muse/chat_context.py: pre_process() appends conversation memory,
location/health instructions, awareness-conditional blocks, and
behavioral defaults to the user instruction
- tests/test_chat_context.py: 12 tests covering all injection paths
and graceful degradation