perf: optimize doc-maintainer token usage (~27% reduction) (#1645)

Implements recommendations from token optimization issue #1643:

1. Remove unused github: toolset — the agent uses bash for all git/file
   ops and safeoutputs for PR creation. The 22 GitHub MCP tools were
   loaded but never called, adding ~13,200 tokens/turn to the system
   prompt. Estimated savings: ~422K tokens/run (~14%).

2. Pre-compute git changes and doc file list in steps: block — eliminates
   the agent's discovery phase (3-5 turns of running git log, find).
   Step outputs are injected into the prompt via template variables.
   Estimated savings: ~190K tokens/run (~8%).

3. max-turns guard — not implemented because the copilot engine does not
   support this feature. The existing timeout-minutes: 15 serves as the
   cost cap.

Expected impact: ~$3.91 → ~$2.55/run (-35% cost), 3,003K → ~2,200K
tokens/run (-27%), 22 → 0 GitHub tools loaded (-100%).

Closes #1643

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: lpcox