Add import-graph / call-graph analysis to domain and hotspot detection

[ivklgn]

Problem

/archcore:init detection is heuristic: plugins/archcore/skills/init/lib/detect-hotspots.md ranks by lines-of-code, test ratio and grep-style fan-in, and detect-domains.md uses directory conventions. Without a real import/call graph, hotspot ranking and domain boundaries are approximations.

Expected result

Real dependency-edge analysis (import/call graph, via per-language parsers) feeding hotspot ranking (true fan-in) and domain-boundary detection.

Impact

Sharper, more accurate first-day maps for larger and polyglot codebases — but large and language-specific, so best scoped as a design spike plus a one-language pilot rather than a single change.

Proposed approach

• Design spike: graph model + integration points into detect-hotspots/detect-domains.
• Pilot one language, then add parsers incrementally.

Acceptance criteria

• A pilot language builds a real import graph that measurably improves hotspot/domain output vs. the heuristics.

References

Design rationale: .archcore/plugin/bootstrap-scale-modes.plan.md and magic-first-day-init.plan.md (both "Out of scope"). Roadmap-optional. Code: plugins/archcore/skills/init/lib/detect-{hotspots,domains}.md.