Add snapshot/harness tests for the context skill with fixture .archcore repos

[ivklgn]

Problem

/archcore:context is a job-critical skill (it surfaces the rules/ADRs/specs/patterns that apply before an edit), but it shipped with no output tests — only the structural test/structure/skills.bats exists, and there is no test/fixtures/context/. Its grouping, truncation, guide-routing and classification behavior is entirely unguarded.

Expected result

2-3 fixture .archcore/ repos plus a harness that runs the skill and snapshots its grouped markdown output, covering the three scope modes (path / topic / pickup), the Reference section (doc/rfc/orphan-guide surfacing), empty-header suppression, guide routing and the classification footer.

Impact

Locks the behavior of a primary user-facing surface so ranking and formatting changes are reviewable and cannot silently regress.

Proposed approach (small PRs)

• Fixture-repo scaffold + harness.
• Path-mode snapshot (grouping, top-5 truncation, empty-header suppression, guide routing, footer).
• Topic-mode and pickup-mode snapshots.

Acceptance criteria

• Each mode produces output matching its snapshot against the fixtures.
• A change to grouping/truncation/routing that alters output fails the snapshot.

References

Design rationale: .archcore/plugin/context-skill-implementation.plan.md (deferred tests). Code: plugins/archcore/skills/context/SKILL.md, test/structure/skills.bats, test/fixtures/.