[skyler-zbt]

Summary

This change adds task-focused mcpp API skills for AI agents, covering commands,
mcpp.toml configuration and public build-program interfaces.

• Organizes 34 skills into nine categories under .agents/skills/mcpp-api-skills/,
  with English instructions and references and bilingual README navigation.
• Adds an interface lookup, a source declaration inventory and local validation tools.
• Updates the contribution guide to require affected skills to be maintained
  alongside public-interface changes.

Closes #876

Criteria

• A detached copy of the suite passes structural validation. Each leaf skill
  includes its required references without depending on other skill directories.
• The checker rejects duplicate skills, unknown interface owners, missing resources,
  escaping paths and broken links. Source comparison reports added, removed or
  changed declarations.
• Examples match implemented behavior, including configuration scope, defaults
  and platform limitations.

Checker acceptance tests are in the suite's tests/test_check_skills.py and were
run locally with Python; they are not integrated into project CI. CLI smoke checks
use an installed binary. mcpp was not rebuilt from source, and its full C++ and E2E
suites were not run. The source baseline and validation scope are documented in
the suite's maintenance/verification.md.

Intersections

New rule or feature	Invariant it crosses	Test at the crossing
Independent leaf skills	Required local resources remain accessible after copying	Detached-copy tests and tests for relative references outside a skill
Interface catalog and checker	Skill ownership, resource references and source declarations remain consistent	Tests for duplicate names, unknown owners, missing resources and declaration extraction
Skill maintenance requirement	Public-interface changes update usage guidance; internal refactors do not require rewriting skill text	Manual review of scope; no automated policy check

Compatibility

This change does not alter mcpp commands, manifest parsing or runtime behavior.
Existing projects require no migration, and existing skills remain available.
The contribution guide adds a requirement to maintain affected skills when
public interfaces change.

Checks before merging

• Documentation style, structure and version-pin checks pass: check_docs_style.sh, check_docs_structure.sh and check_version_pins.sh.
• python .github/tools/check_workflow_assertions.py passes.
• No commit on the branch carries an attribution trailer.
• Suite structure checks, source declaration checks and all 16 Python tests pass.
• Metadata validation passes for 34 skill entrypoints, and all 20 TOML examples parse.
• All 10 CLI smoke checks pass.
• If squash merging is used, the merge has an explicit subject and body.