feat(architect): two-phase read-only tool access (#193 PR 3)

[Abernaughty]

Summary

Third and final PR for #193 — gives the Architect optional read-only tool access via a two-phase flow:

• Phase 1 (unchanged): no-tools LLM call; preserves existing cost profile for simple tasks.
• Sufficiency gate (_blueprint_is_sufficient): rejects empty target_files, placeholder paths (path/to/..., angle brackets, TODO/TBD/unknown), and instructions shorter than 20 chars.
• Phase 2 (new, conditional): binds READONLY_TOOLS (filesystem_list, filesystem_read, github_read_diff) via _run_tool_loop, capped at MAX_ARCHITECT_TOOL_TURNS (default 4), then reparses the Blueprint.
• Fallback: if Phase 2 fails to parse, return the Phase 1 Blueprint so the outer retry loop can still make progress.
• Phase 2 tool calls are logged under the architect agent in result["tool_calls_log"]; Phase 1 output is left untouched when sufficient.

This closes the last gap for the self-dev gate (#113): when the Planner pre-fetches a GitHub issue and hands it to the Architect, the Architect can now actually verify which files exist before emitting the Blueprint.

Test plan

• New tests/test_architect_two_phase.py (16 tests, all pass):
  • 7 heuristic tests for _blueprint_is_sufficient
  • 6 behavioral tests (Phase 1 sufficient skips Phase 2; insufficient no-tools falls through; insufficient escalates; Phase 2 tool calls logged; Phase 1 unparseable → Phase 2 rescues; both fail → FAILED)
  • 1 max-turns test (Phase 2 exhausts turns → falls back to Phase 1 Blueprint with BUILDING status)
  • 2 integration tests simulating Fix-issue-bug: Terminal panel drag-to-resize breaks after ~1/3 screen height #113 flow with Planner-prefetched GitHub context
• uv run ruff check src/ tests/ clean
• Full regression suite: 1097 passed, 2 skipped, 1 pre-existing unrelated failure (test_workspace::test_from_env_defaults env-var leak)
• CI green

Refs #193

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