Add library list command to enumerate available artifacts

[williamthorsen]

Epic: #576 · Depends on: nothing (reads the content library; independent of the declaration format and resolver) · Priority: do first

Problem

The content library's artifacts — rulebooks, skills, and subagents — are discoverable only by browsing the source tree. There's no command to list what's available with its metadata.

Context

Three content subtrees, resolved via resolveContentDir(), each with a distinct frontmatter schema:

• Rulebooks (content/guidance/rulebooks/*.md): slug, description, delivery, version — validated by RulebookFrontmatterSchema.
• Skills (content/skills/*/SKILL.md): name, description, user-invocable.
• Subagents (content/subagents/*.md): name, description, tools, maxTurns.

The package README documents only preferences.yaml — it has no CLI commands section today.

Proposed solution

Add a top-level library command with a list subcommand (new src/commands/library-list.ts), wired into cli.ts following the existing generate <target> two-token pattern. Enumerate each subtree via resolveContentDir(), parse each artifact's frontmatter, and print an aligned plain-text table — one row per artifact.

Columns: type, slug, delivery, description, normalized across the heterogeneous schemas:

• type — rulebook / skill / subagent, prefixed with an identifying emoji: 📕 rulebook, 🪄 skill, 🤖 subagent.
• slug — the stable handle: the rulebook slug, or the skill/subagent name.
• delivery — the rulebook's delivery value (e.g. ambient, skill); skill for skills and subagent for subagents. Accurate per kind, never invented.
• description — the frontmatter description, last column, wrapped with a hanging indent so continuation lines align under the description column; full text preserved. Wrap width is the terminal width at a TTY, a fixed width when piped (keeping output deterministic for tests).

No version column: the library holds one version of each artifact and only the latest is usable, so a version number aids no selection and would be empty for all but rulebooks.

Reuse RulebookFrontmatterSchema to parse rulebooks; read name/description from skill and subagent frontmatter via the existing parseFrontmatter + YAML parse. Rows ordered by type, then slug.

Document the command in printUsage() and add a ## Commands section to the README covering the full CLI surface, so the new command lands documented consistently rather than as the only documented command.

Out of scope

• Collections — the epic's fourth artifact kind (Collections + transitive dependency resolution #856) — don't exist in the library yet; this command enumerates the three present kinds. Structure enumeration as a per-kind descriptor list so adding collections later is a localized, one-entry change. (Reserve 📦 for collections when Collections + transitive dependency resolution #856 wires it.)
• Harness-specific artifacts under _harnesses/.

Acceptance criteria

Must have

• codeassembly-agents library list prints, for each rulebook, skill, and subagent in the shared library, a row with type (emoji + label), slug, delivery, and description.
• slug is the rulebook slug or the skill/subagent name; delivery is the rulebook's delivery value, or skill/subagent for those kinds respectively.
• Long descriptions wrap with a hanging indent that aligns continuation lines under the description column.
• Support directories (_data, _harnesses, _partials) and dotfiles are excluded from enumeration.
• Output is deterministically ordered by type, then slug.
• library list appears in printUsage(), and the README gains a ## Commands section documenting the CLI.
• New behavior is covered by tests.