feat: Expose QualitySchema via `gaze schema quality` and add positional arg support

[jflowers]

Summary

Wire the existing QualitySchema constant into the gaze schema CLI command by adding positional argument support. This is the core bug fix from #118 — QualitySchema is a 248-line, tested constant that no user can access.

Parent issue: #118

Problem

• gaze schema only prints report.Schema (the analyze schema)
• QualitySchema exists at internal/report/schema.go:196, is maintained and tested (4 test functions), but has no CLI exposure
• The gaze schema help text is misleading — says "Gaze analysis output" when it only covers analyze
• README says "JSON output conforms to documented schemas" (line 188) — overstates coverage

Proposed Changes

1. CLI: Add positional arg to newSchemaCmd()

Change cmd/gaze/main.go:412-425:

• cobra.NoArgs → cobra.MaximumNArgs(1)
• Add ValidArgs: []string{"analyze", "quality"} for tab completion
• Default to analyze when no argument given (backward compatible)
• Dispatch to report.Schema or report.QualitySchema based on arg

2. Help text updates

• Short: "Print JSON Schemas for Gaze output formats"
• Long: List available schema names and explain usage

3. Documentation updates

• README.md:188 — clarify which outputs have formal schemas
• docs/reference/cli/schema.md — update synopsis, examples, argument list
• docs/reference/json-schemas.md:354-362 — remove "not exposed via CLI" note

4. Tests

• TestSchemaCmd_NoArgs — backward compatibility (prints analyze schema)
• TestSchemaCmd_AnalyzeArg — gaze schema analyze returns report.Schema (verify $id)
• TestSchemaCmd_QualityArg — gaze schema quality returns report.QualitySchema (verify $id)
• TestSchemaCmd_InvalidArg — clear error on invalid argument
• TestSchemaCmd_AllSchemasAreValidJSON — table-driven JSON validity check

Constraints

• Bare gaze schema MUST continue printing the analyze schema (backward compatibility)
• No new schema constants needed — QualitySchema already exists
• Requires a spec (modifies production code + CLI behavior)