Two issues with headless new-milestone:
1. Default 300s timeout is too short — codebase investigation + artifact
writing for a new milestone regularly exceeds 5 minutes. Bumped to
600s (10 min) when the default hasn't been explicitly overridden.
2. The discuss-headless prompt's 'Investigate' step had no guidance on
how much time to spend scouting. LLMs would exhaustively explore the
codebase (50+ tool calls) before writing any artifacts, running out
of time. Added 'brief' qualifier and 5-6 tool call budget with a
note that the research phase does deeper investigation later.
The commit_docs: false preference is already respected — the prompt
correctly says 'Do not commit' when commit_docs is false, and
ensureGitignore idempotently skips when .gsd/ is already in .gitignore.
Fixes#1227
99fdaa6ad8