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/types.ts

@@ -0,0 +1,53 @@
+/**
+ * Skills system type definitions.
+ *
+ * Skills are modular capability packages that extend Flynn's abilities.
+ * Each skill lives in its own directory with a manifest.json and SKILL.md.
+ */
+
+/** Three tiers of skills: shipped with Flynn, installed via CLI, or user-created. */
+export type SkillTier = 'bundled' | 'managed' | 'workspace';
+
+/** Requirements that must be met for a skill to be available. */
+export interface SkillRequirements {
+  /** OS platforms the skill supports (e.g., ['linux', 'darwin']). Empty = all. */
+  os?: string[];
+  /** Binaries that must exist in PATH (e.g., ['git', 'docker']). */
+  binaries?: string[];
+  /** Environment variables that must be set (e.g., ['GITHUB_TOKEN']). */
+  env?: string[];
+}
+
+/** Manifest for a skill (manifest.json). */
+export interface SkillManifest {
+  /** Unique skill name (e.g., 'git', 'web-search'). */
+  name: string;
+  /** Human-readable description. */
+  description: string;
+  /** Semantic version string. */
+  version: string;
+  /** Author name or identifier. */
+  author?: string;
+  /** Skill tier. */
+  tier: SkillTier;
+  /** System requirements for availability gating. */
+  requirements?: SkillRequirements;
+  /** Tool names this skill provides (informational — not enforced). */
+  tools?: string[];
+  /** npm/system dependencies needed. */
+  dependencies?: string[];
+}
+
+/** A loaded skill ready for use. */
+export interface Skill {
+  /** The parsed manifest. */
+  manifest: SkillManifest;
+  /** Content of SKILL.md — injected into the system prompt. */
+  instructions: string;
+  /** Absolute path to the skill directory. */
+  directory: string;
+  /** Whether the skill's requirements are met on this system. */
+  available: boolean;
+  /** If not available, the reason(s) why. */
+  unavailableReasons?: string[];
+}