fix(agents): claude follows CLAUDE_CONFIG_DIR (Issue #31, review)

CLAUDE_CONFIG_DIR moves claude's whole config root, sessions/ included, so a
live session started under it was not seen and a stopped one left
file-history and the rest behind. The config root is now CLAUDE_CONFIG_DIR,
else <home>/.claude; the purge and residual scans look at ctx.claude_dir and
the root's own projects/, each once. The test guard drops the variable for
every test like the other agent-home variables (test_issue55 l1 requires
every *_DIR the adapters read to be accounted for).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/_agent_home_guard.py

@@ -18,7 +18,7 @@ import tempfile
 
 # The pane-inherited variables conftest._isolate_agent_homes drops for every
 # test, so HOME (which the tests set) governs, as it does in a plain shell.
-DROPPED_VARS = ("HOME_DIR", "CLAUDE_PROJECT_DIR", "CODEX_HOME", "LOCAL_BIN")
+DROPPED_VARS = ("HOME_DIR", "CLAUDE_PROJECT_DIR", "CLAUDE_CONFIG_DIR", "CODEX_HOME", "LOCAL_BIN")
 # Read by the code under test but left alone, on purpose: HOME is where the
 # real herdr tests and the shim bootstrap find the server socket, and it is
 # what the suite's own tests set for an isolated home; XDG_CONFIG_HOME and