feat(skills): add five task-focused agent skills alongside the router skill

[dmautz1]

Summary

Expands the plugin from one skill to six: the existing tablespec skill becomes a true routing layer, and five new sibling skills carry the mechanics for the workflows an AI is most likely to get wrong without instructions. Every command, flag, and API symbol in the new skills was verified against source before writing.

New skills (each a single skills/<name>/SKILL.md):

• tablespec-umf-authoring — split-YAML layout rules (including the load-bearing column:/validations: file nesting), CLI mutation commands, convert semantics
• tablespec-pipeline — Path A/B bootstrap, compile_umfs, the pinned artifact tree, run_backbone stage ordering and Databricks gating
• tablespec-validation — the raw/ingested staging model, validation-sync/preview/apply-response, domain-types-before-sync ordering
• tablespec-sql-plans — derivation candidates, all five base_table_strategy variants, DuckDB↔Spark byte-parity rules
• tablespec-profiling-app — no-Spark-in-container reflection via umf_from_information_schema, provision/configure/deploy, the root-vs-subfolder deploy trap

Router refactor (skills/tablespec/SKILL.md): task→skill routing table, full CLI index, public facade enumerated, stale "no single-call bootstrap facade yet" claim fixed.

Plumbing:

• .claude/skills/ and .agents/skills/ symlinks committed (with .gitignore negations) so the skills also load from a plain repo clone without installing the plugin
• tests/docs/test_plugin_manifests.py parameterized over skills/*/SKILL.md (frontmatter, forbidden-language, references-dir, and relative-symlink checks per skill) plus a pinned expected-skill-set test
• README plugin section lists all six skills
• No manifest edits needed (skills: \"./skills/\" auto-discovers); the version stays 0.0.3 — the next release must bump all three manifest versions to the tag per the version-guard CI job

Stale docs fixed (surfaced by the source verification while writing the skills):

• docs/guide/great-expectations.md rewritten against the real APIs (extract_from_checkpoint / UmfToGxMapper.map_columns never existed; baseline generation deliberately excludes column existence/type checks)
• Inert tablespec preview --against/-v flags removed from the CLI and website CLI reference (bodies never read them; FEAT-023 still tracks the dry-run feature)
• Dead OUTPUT_VOLUME_CATALOG env entry dropped from both app manifests; root app.yaml now declares the PROFILER_METADATA_* inputs profiler/config.py actually resolves
• apps/data-profiling/README.md rewritten from milestone-1/stub-era content to the shipped state, keeping the D2D Sharing runbook as a future-use appendix

Test plan

• uv run pytest tests/docs/ — 58 passed (manifest tests now cover all six skills)
• uv run pytest tests/unit/test_preview.py tests/unit/test_cli_validation_commands.py tests/unit/test_apply_response.py — passed
• make app-smoke — 62 passed (covers config/manifest resolution)
• ruff check / ruff format --check clean on changed Python
• All six skills load and trigger in a live Claude Code session in the repo clone
• Post-merge release: bump the three manifest versions and smoke-test claude plugin install tablespec@tablespec (and a Codex install) to confirm six skills load for plugin installers

🤖 Generated with Claude Code