feat(skills): add per-task skill spec auto-detection and injection

Replace external inject-spec.py hook with built-in zero-config skill
detection in codeagent-wrapper. The system auto-detects project type
from fingerprint files (go.mod, package.json, etc.), maps to installed
skills, and injects SKILL.md content directly into sub-agent prompts.

Key changes:
- Add DetectProjectSkills/ResolveSkillContent in executor/prompt.go
- Add Skills field to TaskSpec with parallel config parsing
- Add --skills CLI flag for explicit override
- Update /do SKILL.md Phase 4 with per-task skill examples
- Remove on-stop.py global hook (not needed)
- Replace inject-spec.py with no-op (detection now internal)
- Add 20 unit tests covering detection, resolution, budget, security

Security: path traversal protection via validSkillName regex,
16K char budget with tag overhead accounting, CRLF normalization.

Generated with SWE-Agent.ai

Co-Authored-By: SWE-Agent.ai <noreply@swe-agent.ai>

codeagent-wrapper/internal/executor/executor.go

@@ -337,6 +337,16 @@ func DefaultRunCodexTaskFn(task TaskSpec, timeout int) TaskResult {
 		}
 		task.Task = WrapTaskWithAgentPrompt(prompt, task.Task)
 	}
+	// Resolve skills: explicit > auto-detect from workdir
+	skills := task.Skills
+	if len(skills) == 0 {
+		skills = DetectProjectSkills(task.WorkDir)
+	}
+	if len(skills) > 0 {
+		if content := ResolveSkillContent(skills, 0); content != "" {
+			task.Task = task.Task + "\n\n# Domain Best Practices\n\n" + content
+		}
+	}
 	if task.UseStdin || ShouldUseStdin(task.Task, false) {
 		task.UseStdin = true
 	}