fix: one convention for phase report filenames

[MCKRUZ]

Closes #36.

Stacked on #37 — based on fix/complete-fix-3-receipts, so this diff shows only its own change. Merge #37 first; GitHub will retarget this to master automatically.

The problem

Three naming conventions for the same generated files, shipping together:

Source	Spelling	Phase 9 example
phases/*.md	zero-padded	phase09-report.html
phase-registry.yaml artifacts.optional	unpadded	phase9-report.html
docs/commands.md	registry slug	09-monitoring-report.html

03-foundation.md alone used both the padded and unpadded forms.

Why it actually mattered

Not just untidiness — two things were broken:

1. commands/sdlc-gate.md pre-checks the visual report by slug. A team following the phase definition wrote phase09-visual.html; the gate looked for 09-monitoring-visual.html and reported it missing.
2. The registry's artifacts.optional entries were dead. They could never match a real file produced under either of the other two conventions.

The fix

All 38 references now use <slug>-report.html / <slug>-visual.html.

The slug wins because it is what generate_phase_report.py is actually invoked with (its own docstring example is build-report.html), and it is the only form that survives the non-numeric build and close phases — phase4-report.html has no meaning.

The rename was applied mechanically with the mapping derived from the registry rather than hardcoded, so it cannot silently disagree with the registry it was meant to match.

Test plan

• Full suite: 538 passed, 6 skipped
• Zero phase\d+-(report|visual)\.html references remain anywhere
• The 17 surviving names reviewed by hand and all on the slug convention
• New guard test_phase_reports_use_the_registry_slug_convention mutation-tested — reintroduced phase09-visual.html, confirmed it fails, restored

Note

Phase 2 has no -visual counterpart. That is deliberate, not a gap: its diagram artifact is separately named .sdlc/reports/architecture-diagrams.html. Left as-is.

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