fix: fail closed when no safe export destination exists; polish salvage edges

- _profile_export_directory(): when the managed store, the home-sibling
  store, AND the temp dir all resolve inside Git checkouts, raise a clear
  ValueError instead of warning and proceeding — a stderr warning would not
  stop a scripted export from staging a secret-bearing archive in a source
  tree, which is the exact #92457 incident class. All three callers already
  surface ValueError cleanly (CLI/TUI print Error: + exit, API returns 400).
- .dockerignore: drop the /default.tar.gz line made redundant by the global
  *.tar.gz pattern this PR adds.
- hermes profile export -o help text: stop advertising the old
  <name>.tar.gz cwd default.
- Tests: cwd-in-unrelated-checkout topology (the second production shape
  from the blocking review) and the fail-closed path. Mutation-checked:
  both fail on the pre-fix helper.

hermes_cli/profiles.py

@@ -2154,13 +2154,14 @@ def _profile_export_directory() -> Path:
     for candidate in candidates:
         if not _inside_git_checkout(candidate):
             return candidate
-    logger.warning(
-        "Every managed export destination resolves inside a Git checkout; "
-        "falling back to %s. Pass an explicit output path to avoid staging "
-        "profile archives in a source tree.",
-        export_dir,
+    # Fail closed: writing a secret-bearing archive into a source tree is the
+    # incident this helper exists to prevent (#92457). A warning on stderr
+    # would not stop a scripted export from recreating it.
+    raise ValueError(
+        "No safe automatic export destination: every candidate directory is "
+        "inside a Git checkout. Pass an explicit output path outside the "
+        "checkout (e.g. -o /path/outside/repo/profile.tar.gz)."
     )
-    return export_dir
 
 
 def get_profile_export_path(name: str, *, timestamp: Optional[str] = None) -> Path: