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/utils/strings_test.go

@@ -19,7 +19,7 @@ func TestTruncate(t *testing.T) {
 		{"zero maxLen", "hello", 0, "..."},
 		{"negative maxLen", "hello", -1, ""},
 		{"maxLen 1", "hello", 1, "h..."},
-		{"unicode bytes truncate", "你好世界", 10, "你好世\xe7..."},  // Truncate works on bytes, not runes
+		{"unicode bytes truncate", "你好世界", 10, "你好世\xe7..."},    // Truncate works on bytes, not runes
 		{"mixed truncate", "hello世界abc", 7, "hello\xe4\xb8..."}, // byte-based truncation
 	}