feat(agent/agentcontextconfig): discover skills from ~/.coder/skills (#25271)

The default skills lookup only scanned the project-relative
.agents/skills directory, so personal skills had to be repeated
per project or wired in via CODER_AGENT_EXP_SKILLS_DIRS. Now the
default is the comma-separated list ~/.coder/skills,.agents/skills,
which lets discoverSkills's existing first-occurrence-wins policy
prefer home-scoped skills over project ones with the same name.

The change is additive when ~/.coder/skills is absent
(missing directories are silently skipped in discoverSkills) and
unaffects users who set the env var explicitly.

Closes CODAGT-403

agent/agentcontextconfig/api.go

@@ -60,10 +60,14 @@ var skillNamePattern = regexp.MustCompile(
 
 // Default values for agent-internal configuration. These are
 // used when the corresponding env vars are unset.
+//
+// DefaultSkillsDir is a comma-separated list so home-scoped
+// skills override project-scoped ones with the same name
+// (discoverSkills picks the first occurrence per skill name).
 const (
 	DefaultInstructionsDir  = "~/.coder"
 	DefaultInstructionsFile = "AGENTS.md"
-	DefaultSkillsDir        = ".agents/skills"
+	DefaultSkillsDir        = "~/.coder/skills,.agents/skills"
 	DefaultSkillMetaFile    = "SKILL.md"
 	DefaultMCPConfigFile    = ".mcp.json"
 )