feat(blast-radius): PR 4 — specialist consumption + flip default to on

[ni5h4nt]

Summary

PR 4 of the blast-radius initiative — the one that actually changes reviewer behavior. Three coordinated changes; the flag flip is the last commit so a regression revert is exactly one commit.

1. Real base/head content via git_provider. New GitHostAdapter.get_file_content + GitHubAdapter impl + fetch_file_contents_activity (rename-aware: uses old_path for base SHA, path for head; skip-on-error rather than coercing exceptions to empty content).
2. Specialists consume impact_context. coordinator.py and analyzer.py both render a ## Impact Context markdown section into their user prompts via the shared render_impact_context helper in a new neutral module app/services/review/impact_prompt.py. Every specialist's system prompt is augmented with a standing rule about confidence tiers (EXTRACTED/INFERRED = definitely impacted; AMBIGUOUS = possibly impacted, lower severity by one level if a finding depends on one). Done in the coordinator/analyzer, not per-specialist file.
3. Default flag flipped to True. Last commit. Revert just fbcf61c-equivalent to disable the feature without touching prompt or git-provider code.

Stacked on PR 3 (PR #4 in this repo). GitHub will auto-retarget the base branch when PR 3 merges.

Spec: docs/superpowers/specs/2026-05-02-blast-radius-pre-filter-design.md
Plan: docs/superpowers/plans/2026-05-04-blast-radius-pr4-specialist-consumption.md (lives on the umbrella branch)

What lands (8 commits, 20 files, +955/-30)

• app/services/git_provider.py + app/services/github_adapter.py — get_file_content(owner, repo, path, sha) -> Optional[str]. None for 404 / directory / non-file / decode failure.
• app/services/review/impact_prompt.py (NEW, neutral helper module) — _STANDING_AMBIGUITY_RULE, public render_impact_context(impact), public augment_system_prompt(base). Exists specifically to break the coordinator <-> analyzer import cycle.
• app/workflows/context_activities.py — fetch_file_contents_activity registered in REGISTERED_ACTIVITIES. Rename-aware. Skip-on-error (genuine 404 -> empty string; exception -> file dropped from output).
• app/workflows/review_workflow.py — fetch_file_contents runs before extract_changed_symbols when the flag is on; threads old_path per file.
• app/services/review/multi_agent_analyzer.py — multi_agent_analyze_file accepts impact_context; _build_context puts it under context["impact"] (legacy integer slot redefined for the symbol-level payload, documented inline).
• app/services/review/coordinator.py — _build_review_prompt renders the new section; Agent(...) constructed with augment_system_prompt(config["prompt"]).
• app/services/analyzer.py — single-agent analyze_file accepts impact_context; new _build_user_prompt helper appends render_impact_context(...) when non-None; system prompt augmented identically to multi-agent. Lazy-imports the helpers to break the package-init cycle (documented inline).
• app/workflows/activities.py — analyze_file_activity forwards input.impact_context to BOTH paths.
• app/core/config.py — blast_radius_enabled: bool = True (was False). Last commit.

Architecture decisions

• Standing rule lives in impact_prompt.py, not per-specialist. Seven specialist subclass files would otherwise need identical edits; augment_system_prompt(base) does it in one place.
• get_file_content returns Optional[str] — None for genuine 404 (added/removed/directory), exception propagates for transient errors. The activity coerces None to "" so the chunker runs on added/removed files; exceptions trigger per-file skip rather than fabricating empty content.
• context["impact"] slot redefined. Pre-PR-4 it carried an integer impact-radius from the legacy ImpactAnalyzer (no consumer reads it). PR 4 reuses the slot for the symbol-level BlastRadius payload.
• Lazy imports in analyzer.py for the impact_prompt helpers. The neutral module itself has no review-package deps, but app/services/review/__init__.py eagerly imports from coordinator.py (which imports from analyzer.py), recreating the cycle on submodule import. Lazy imports fix it cleanly without a wider __init__.py refactor.

Test plan

• get_file_content happy path + 404 + directory + empty-file
• fetch_file_contents_activity returns the right per-file payloads + rename-aware base fetch + skip-on-error + worker-registration object-identity
• Workflow: flag-off skips fetch_file_contents; flag-on runs fetch_file_contents < extract_changed_symbols < compute_blast_radius < analyze_file_activity
• _build_context carries impact_context under "impact"
• render_impact_context covers all sections + graph-unavailable case
• Coordinator constructs Agent with system_prompt containing the standing AMBIGUOUS rule (end-to-end wiring)
• Single-agent _build_user_prompt renders the same section
• analyze_file_activity forwards impact_context to multi-agent path
• Default flag is True; workflow blast-radius tests still pass
• uv run ruff check clean across all 11 touched Python files
• 50/50 targeted Python tests pass

Rollback plan

Revert just commit feat(blast-radius): flip blast_radius_enabled default to True. Everything else stays — workflow phase still runs, prompts still know how to render impact context, but every Settings() defaults to False again so the phase short-circuits.