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/config/schema.ts

@@ -50,6 +50,15 @@ const hooksSchema = z.object({
   silent: z.array(z.string()).default([]),
 }).default({});
 
+const skillsSchema = z.object({
+  /** Directory for user-created workspace skills. */
+  workspace_dir: z.string().optional(),
+  /** Directory for managed (installed) skills. Defaults to ~/.flynn/workspace/skills. */
+  managed_dir: z.string().optional(),
+  /** Directory for bundled skills shipped with Flynn. */
+  bundled_dir: z.string().optional(),
+}).default({});
+
 const mcpServerSchema = z.object({
   name: z.string(),
   command: z.string(),
@@ -68,6 +77,7 @@ export const configSchema = z.object({
   models: modelsSchema,
   backends: backendsSchema.default({}),
   hooks: hooksSchema.default({}),
+  skills: skillsSchema.default({}),
   mcp: mcpSchema.default({ servers: [] }),
 });