Codec Markdown + Unified Inventory (.md with YAML frontmatter)

[JonathLima]

What to build

Replace the dual JSON+Markdown inventory format with a single .md\ file using YAML frontmatter (camelCase, typed fields) + readable Markdown body. This is a clean break — no backward compatibility with legacy JSON states.

Scope

• \Cargo.toml: swap \serde_yaml\ for \yaml_serde 0.10\ via package rename (zero import changes, maintained fork by YAML org)
• \inventory.rs: rewrite \write_inventory_at\ to emit .md\ with frontmatter;
  ead_inventory_at\ to parse frontmatter; \inventory_path\ returns .orquestra/skills_inventory.md; remove \inventory_md_path\
• YAML 1.1 protection: #[serde(deserialize_with)]\ on \SkillInventory\ string fields — reject ambiguous YAML booleans (
  ame: yes) with an error asking for quotes (no silent coercion)
• No legacy JSON fallback — pre-publication clean break
• \lib.rs: remove \inventory_md_path\ from exports, keep \scan_one_skill\ export
• Update tests: YAML round-trip, YAML 1.1 rejection, write/read cycle

Acceptance criteria

• \cargo test -p orquestra-skills\ passes
• \cargo test --workspace\ passes (no new regressions)
• \write_inventory_at\ produces .orquestra/skills_inventory.md\ with valid YAML frontmatter + Markdown body
• 
  ead_inventory_at\ parses the file back correctly
• 
  ame: yes\ in YAML frontmatter raises an error (not silently coerced to boolean)
• No .orquestra/skills_inventory.json\ is written anymore

Blocked by

None — can start immediately.