feat: Create CRAPSchema constant and expose via `gaze schema crap`

[jflowers]

Summary

Create a formal JSON Schema (Draft 2020-12) for gaze crap --format=json output and wire it into the gaze schema command. This is the feature request portion of #118.

Parent issue: #118
Depends on: #140 (positional arg support must exist first)

Problem

The CRAP JSON output is the most likely to be machine-consumed by CI scripts and dashboards, but has no formal JSON Schema. The docs/reference/json-schemas.md:119 explicitly notes: "The CRAP JSON output is not covered by a formal embedded schema."

Scope Decisions Required

1. Output mode scoping

gaze crap produces two different JSON structures:

• Normal mode: crap.Report — scores []Score + summary Summary
• Baseline comparison mode (--baseline): crap.ComparisonResult — deltas, new_functions, removed_functions, comparison

Decision needed: Does the schema cover both output modes, or only the standard crap.Report? Options:

• (a) Schema covers only crap.Report; document that --baseline produces a different structure
• (b) Schema uses oneOf to cover both shapes
• (c) Two separate schemas: CRAPSchema and CRAPComparisonSchema

2. Forward compatibility

Consider adding additionalProperties: true to allow additive field changes without breaking downstream validation. The CRAP output has gained 6 new fields across recent releases.

Proposed Changes

1. New constant in internal/report/schema.go

CRAPSchema constant (~200-300 lines) covering crap.Report:

• Score struct: 14 JSON fields (8 optional with omitempty)
• Summary struct: ~15 JSON fields (8 optional)
• RecommendedAction struct: 10 JSON fields
• Enum types: Quadrant (4 values), FixStrategy (4 values)
• Follow existing pattern: Draft 2020-12, $id URL, $defs for shared types, description on every field

2. CLI wiring

Add "crap" to ValidArgs in newSchemaCmd() and dispatch to report.CRAPSchema.

3. Tests (round-trip, not hand-crafted)

• TestCRAPSchema_Compiles — schema is valid Draft 2020-12
• TestCRAPSchema_ValidatesMinimalOutput — minimal Report with one Score
• TestCRAPSchema_ValidatesFullOutput — all optional fields populated
• TestCRAPSchema_ValidatesEmptyScores — empty scores array
• TestCRAPWriteJSON_ValidAgainstSchema — actual WriteJSON() output validated against schema (critical round-trip test)

4. Documentation

• docs/reference/json-schemas.md — add annotated CRAP schema section
• docs/reference/cli/schema.md — add crap to argument list and examples

Risks

• Maintenance ratchet: Every future field addition to Score/Summary requires a schema update
• Schema drift: Manually-maintained Go string constants have no automated drift detection against struct tags
• Baseline mode: If not covered by the schema, users may be confused when --baseline output fails validation

Note on report output

Per the review council analysis on #118, gaze report --format=json should NOT get a schema. It is explicitly documented as "an internal format" (json-schemas.md:352), uses json.RawMessage fields, and CI integration uses threshold flags rather than schema validation.