docs(skills): add graphile-meta skill; ignore docs in CI; drop skill zips

[pyramation]

Summary

Three related changes:

1. New graphile-meta skill (.agents/skills/graphile-meta/SKILL.md)
documenting the _meta schema-introspection plugin (MetaSchemaPlugin /
MetaSchemaPreset in graphile-settings). There was no skill for it — the
existing cli-meta / orm-meta skills cover a different thing (the metaschema
API target in constructive-db), not the graphile _meta query. The
skill covers the plugin architecture (two schema hooks, build-time cache), the
full _meta shape, the smart-tag→builder→Meta* type contract, a 6-step recipe
for adding a new metadata block (written around the in-flight scope addition),
and why _meta matters for codegen.

2. CI: ignore docs-only changes. run-tests.yaml had a branches: filter
but no paths filter, so every PR to main/v1 ran the full ~38-job suite —
including markdown-only ones (this PR's first push triggered all of them). Added
paths-ignore to both the push and pull_request triggers:

paths-ignore:
  - '**/*.md'
  - '.agents/skills/**'
  - 'docs/**'

Note: GitHub evaluates paths-ignore from the base branch's workflow, so this PR
still runs full CI (it also edits the workflow + deletes files); the skip takes
effect for future docs-only PRs after merge.

3. Removed the committed skill .zip bundles. The 6 curated skills each
shipped a .zip next to their dir. Nothing in the repo references or consumes
them (no build script / workflow / tooling) — they were a one-off manual artifact
from the "add 6 tooling skills" commit. Deleted all 7 (constructive-cli,
constructive-pnpm, constructive-setup, constructive-testing,
graphile-search, pgpm, and the graphile-meta one I'd just added).

Docs / CI-config only; no runtime code changes.

Link to Devin session: https://app.devin.ai/sessions/819d1e4face6493393be40f35a48af33
Requested by: @pyramation

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