fix(herdr): ensure unique agent name via sha1 truncation and align mock errors

- Replace legacy s[:16]-s[-15:] truncation with 8-char SHA-1 hash suffix in sanitize.py and lib.sh to prevent workspace name collisions
- Align mock Herdr error messages in tests/conftest.py to real Herdr 0.8.0 output format
- Add missing required/invalid_agent_name to early abort regex in lib.sh
- Remove legacy heuristics in lib.sh has-session and unify mock agent lookups
- Add unit tests in tests/test_sanitize_and_mock_errors.py (256/256 passed)

.agents/skills/lib_py/agents/sanitize.py

@@ -1,5 +1,4 @@
-# sanitize.py — Unified agent name sanitization contract for herdr 0.8.0 (E3 & E4)
-
+import hashlib
 import re
 
 def sanitize_herdr_agent_name(name: str) -> str:
@@ -8,7 +7,7 @@ def sanitize_herdr_agent_name(name: str) -> str:
     1. Convert to lowercase.
     2. Replace invalid characters ([^a-z0-9_-]) with '-'.
     3. Ensure starts with a lowercase letter [a-z], prefixing with 'x-' if needed.
-    4. Truncate to maximum 32 characters (16 + '-' + 15).
+    4. Truncate to maximum 32 characters using 8-char SHA-1 hash suffix (s[:23] + '-' + sha1[:8]).
     """
     if not name:
         return "agent"
@@ -23,8 +22,10 @@ def sanitize_herdr_agent_name(name: str) -> str:
     if not s or not s[0].isalpha():
         s = "x-" + s
     
-    # 4. Truncate to 32 chars if needed
+    # 4. Truncate to 32 chars if needed using collision-free SHA-1 hash suffix
     if len(s) > 32:
-        s = s[:16] + '-' + s[-15:]
+        h = hashlib.sha1(s.encode('utf-8')).hexdigest()[:8]
+        s = f"{s[:23]}-{h}"
     
     return s
+