fix(cleanup): remove ghost acp/ package and orphaned .pyc files

The ACP module was properly renamed to A2A in commit ec0b7631 (closing #688),
but the deprecated src/cleveragents/acp/ path was not explicitly excluded from
version control. While __pycache__/ and *.py[cod] are already gitignored
globally, add an explicit .gitignore entry for the deprecated acp/ package path
to prevent accidental re-introduction and document the deprecation per ADR-047.

Verification performed:
- Confirmed src/cleveragents/acp/ does not exist in git (only __pycache__
  artifacts remain on developer machines as untracked build artifacts)
- Confirmed zero tracked .pyc files across the entire repository
- Confirmed zero imports referencing cleveragents.acp in source, test, and
  configuration files
- Confirmed no orphaned .pyc files exist without corresponding .py sources
- All nox sessions pass (coverage: 98.7%)

ISSUES CLOSED: #947

.gitignore

@@ -166,6 +166,9 @@ config.local.toml
 .pabotsuitenames
 PIPE
 
+# Deprecated packages — removed by ADR-047 (ACP → A2A rename)
+src/cleveragents/acp/
+
 # Git worktrees for parallel task branches
 worktrees/
 *.bak