build: consume published quarto-yaml 0.1.0; delete in-tree YAML stack (bd-egcyeym9)

[cscheid]

Summary

Final phase of the diagnostics/YAML extraction epic (bd-egcyeym9). The YAML stack
— quarto-yaml (parser) and quarto-yaml-validation (schema validation) — now
lives in the standalone posit-dev/quarto-yaml workspace and is published to
crates.io at 0.1.0. This PR cuts q2 over to the published quarto-yaml and
deletes the in-tree copies.

Follows the same shape as the foundation cutovers (#348 source-map, #350
error-reporting).

Changes

• Root Cargo.toml: [workspace.dependencies.quarto-yaml] path →
  version = "0.1.0"; dropped [workspace.dependencies.quarto-yaml-validation].
• Consumers: converted the two remaining quarto-yaml path-deps (pampa,
  quarto-config) to { workspace = true }; quarto-core / quarto-lsp-core
  already were.
• Deleted: crates/quarto-yaml, crates/quarto-yaml-validation, and the demo
  binary crates/validate-yaml (the only in-tree consumer of
  quarto-yaml-validation).
• CLAUDE.md: moved quarto-yaml into the Externalized foundation crates
  section; reframed quarto-yaml-validation as published-but-not-a-q2-dep; removed
  the validate-yaml binary entry.

Why q2 keeps only quarto-yaml

quarto-yaml-validation's only in-tree consumer was the validate-yaml demo
binary. With it deleted, q2 has zero dependency on quarto-yaml-validation — it
is published purely for external Posit consumers. q2 continues to depend on the
published quarto-yaml.

Error codes

quarto-yaml-validation ships its error codes unchanged as Q-1-x for 0.1.0
(decided: keep non-breaking now; migrate to namespaced yaml-schema/* origin codes
in 0.2.0).

Snapshot changes

One .snap file is removed as part of deleting crates/quarto-yaml-validation
(…__diagnostic__tests__all_three_formats_snapshot.snap). No q2 snapshot changed
in place — this is purely the crate deletion.

Verification

• cargo build --workspace clean; Cargo.lock (root + wasm) resolves
  quarto-yaml 0.1.0 from the registry (checksum c32ab7b3…);
  quarto-yaml-validation / validate-yaml absent from the lock.
• cargo nextest run --workspace: 9855 passed, 197 skipped.
• Full cargo xtask verify GREEN — all 14 steps incl. the hub-client WASM
  build + hub tests. The WASM crate resolves quarto-yaml transitively (no direct
  dep needed); its standalone Cargo.lock flips 0.7.0 path → 0.1.0 registry.

🤖 Generated with Claude Code