feat(p2,p8,scorecard): schema subcommand, schema file, AGENTS.md frontmatter; fix badge slug + P8 filter

[brettdavies]

Summary

Closes the dogfood gaps surfaced by anc check . at the v0.4.0 release candidate, plus the silent slug-derivation bug
behind the scorecard's badge URLs. Self-check after this branch: 44 checks, 37 pass, 3 warn, 0 fail, 0 error. Score 93%
(up from 85% at the v0.4.0 RC).

Changelog

Added

• Add anc schema top-level subcommand. Prints the embedded JSON Schema (draft 2020-12) describing the shape of anc check --output json scorecards. Closes the p2-must-schema-print FAIL surfaced by self-check.
• Add schema/scorecard.schema.json committed at the repo root and embedded into the binary via include_str!. Hand-written coverage of the 0.5 shape (Scorecard plus ToolInfo, AncInfo, RunInfo, PlatformInfo, TargetInfo, BadgeInfo, LevelCounts, CoverageSummary, Summary, CheckResultView). Closes the p2-should-schema-file WARN.
• Add YAML frontmatter to AGENTS.md naming the tool, binary, description, homepage, and repository so agent runtimes can index the bundle. Closes the p8-should-bundle-exists WARN.

Changed

• derive_tool_name now follows the four-tier fallback chain command_name -> binary basename -> manifest package name -> directory basename. The old shape returned the project directory basename, producing 404-bound badge URLs for any tool whose registry slug differed from its directory name. anc check . now emits badge_url: https://anc.dev/badge/anc.svg (HTTP 200, matches the site's registry.yaml).
• matches_principle gains the (CheckGroup::P8, 8) arm. Pre-fix, --principle 8 silently filtered out every P8 check because the match table predated the new principle.
• README refreshed for v0.4.0: principle count 7 -> 8 with a P8 row, "Example Output" rewritten against the current 44-check self-check (P2 schema, P6 standard-names + SIGTERM, P8 bundle results), "Three Check Layers" lists Python alongside Rust under Source, JSON sample dropped the stale anc.commit field. README em-dash density scrubbed from 20.5/1000 to 0/1000.
• README "Reporting issues" section folded into "Contributing" as a three-tier intake (signal / proposal / code) that points at the shipped CONTRIBUTING.md for the full breakdown. Cross-repo routing preserved.

Documentation

• Update --principle <PRINCIPLE> doc from (1-7) to (1-8) in both README and src/cli.rs.

Type of Change

• feat: New feature (non-breaking change which adds functionality)

Related Issues/Stories

• Story: n/a
• Issue: n/a
• Architecture: Schema follow-up plan on dev at docs/plans/2026-04-30-002-feat-scorecard-json-schema-plan.md (full schemars-derive implementation; this PR ships the hand-written first cut).
• Related PRs: feat(spec): sync v0.4.0 — add 11 requirement IDs across P1/P2/P4/P6/P8 #50, fix(project): pick newer cargo target binary by mtime #51, feat(prose): import prose pipeline; un-vendor orchestrator; doc-hygiene pass #52, release: v0.4.0 #53

Testing

• Unit tests added/updated
• Integration tests added/updated
• Manual testing completed
• All tests passing

Test Summary:

• New src/main.rs::tests module covers all four tiers of derive_tool_name_inner (command, binary, Cargo.toml manifest, pyproject.toml manifest, directory fallback).
• cargo test: 593 passed, 2 ignored across 7 suites.
• cargo clippy --release --all-targets -- -Dwarnings: clean.
• cargo fmt --check: clean.
• ./target/release/anc check .: 44 checks, 37 pass, 3 warn, 0 fail, 4 skip, 0 error. Score 93%.
• Live URL probe: https://anc.dev/badge/anc.svg returns 200.

Files Modified

Modified:

• AGENTS.md, README.md, src/cli.rs, src/main.rs.
• All five completions/anc.* files (regenerated for the new schema subcommand).

Created:

• schema/scorecard.schema.json.

Renamed:

• None.

Deleted:

• None.

Breaking Changes

• No breaking changes

Deployment Notes

• No special deployment steps required. After merge to dev, cherry-pick the squash commit onto release/v0.4.0 (PR release: v0.4.0 #53) and regenerate CHANGELOG.md before the release tag.