feat: add skills system for extensible capability packages

Implement a three-tier skill system (bundled/managed/workspace) that
extends Flynn's abilities via SKILL.md instructions injected into the
system prompt.

- SkillManifest/Skill types with requirements gating (OS, binaries, env)
- Loader: discovers skills from directories, validates manifests,
  checks system requirements, infers manifest from SKILL.md if missing
- SkillRegistry: holds skills, generates system prompt additions,
  supports override by name (workspace > managed > bundled)
- SkillInstaller: copies/removes skills in managed directory with
  upgrade support
- Config: add skills.workspace_dir, managed_dir, bundled_dir options
- Daemon: loads all skills at startup, injects available skill
  instructions into the system prompt
- Tests: 45 new tests (loader 22, registry 11, installer 12)

src/skills/registry.ts

@@ -0,0 +1,66 @@
+import type { Skill } from './types.js';
+
+/**
+ * SkillRegistry holds loaded skills and generates system prompt additions.
+ *
+ * Skills are keyed by name. Managed/workspace skills may override bundled
+ * skills by registering with the same name.
+ */
+export class SkillRegistry {
+  private skills: Map<string, Skill> = new Map();
+
+  /** Register a skill. Replaces any existing skill with the same name. */
+  register(skill: Skill): void {
+    this.skills.set(skill.manifest.name, skill);
+    console.log(
+      `Skill '${skill.manifest.name}' registered (${skill.manifest.tier}, ${skill.available ? 'available' : 'unavailable'})`
+    );
+  }
+
+  /** Unregister a skill by name. Returns true if the skill existed. */
+  unregister(name: string): boolean {
+    return this.skills.delete(name);
+  }
+
+  /** Look up a skill by name. */
+  get(name: string): Skill | undefined {
+    return this.skills.get(name);
+  }
+
+  /** Return all registered skills. */
+  list(): Skill[] {
+    return Array.from(this.skills.values());
+  }
+
+  /** Return only skills whose requirements are met (available === true). */
+  listAvailable(): Skill[] {
+    return this.list().filter(skill => skill.available);
+  }
+
+  /**
+   * Generate system prompt additions from all available skills.
+   *
+   * Each skill's SKILL.md content is formatted as a markdown section:
+   * ```
+   * ## Skill: <name>
+   * <instructions>
+   * ```
+   *
+   * Returns an empty string if no skills are available.
+   */
+  getSystemPromptAdditions(): string {
+    const available = this.listAvailable();
+    if (available.length === 0) {
+      return '';
+    }
+
+    return available
+      .map(skill => `## Skill: ${skill.manifest.name}\n${skill.instructions}`)
+      .join('\n\n');
+  }
+
+  /** Return the names of all available skills. */
+  getSkillNames(): string[] {
+    return this.listAvailable().map(skill => skill.manifest.name);
+  }
+}