Warn that Auto falls back to prompting when unavailable

Claude Code starts in its prompting mode when auto isn't available for the
session's model or backend, so a headless Auto task can still stall. Say so
in the task editor, the task runner comment, and HOW-TO-USE.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

HOW-TO-USE.md

@@ -484,7 +484,8 @@ the way they did: one that had Full Permissions on becomes **Bypass**, one that
 **Auto** sits between Accept Edits and Bypass: Claude Code's own classifier reviews each action,
 lets routine work through and blocks things that look risky (such as destructive or
 exfiltrating commands) — no prompts either way. Whether it is available depends on your Claude
-Code account and model; see Claude Code's documentation on permission modes.
+Code account, model and backend (local and OpenAI-compatible backends won't qualify). When it
+isn't available, Claude Code quietly starts in its normal prompting mode instead.
 
 ### When a change takes effect
 
@@ -502,7 +503,8 @@ Code account and model; see Claude Code's documentation on permission modes.
 > Scheduled tasks run headless (`claude -p`) and cannot answer a permission prompt. In any mode
 > other than **Auto** or **Bypass**, a task may simply stop early when Claude Code asks for
 > approval. In **Auto**, actions the classifier blocks are denied and the run carries on without
-> them. Its run log records which mode it used.
+> them — but if Auto isn't available for the project's model or backend, Claude Code falls back
+> to prompting and the task can stall the same way. Its run log records which mode it used.
 
 ---
 
@@ -1357,7 +1359,7 @@ with `--dangerously-skip-permissions`. Because the mode travels into the contain
 environment variable, **stop and start the project** after changing it for the scheduler to see the
 change. Remember that a headless run cannot answer a permission prompt, so in any mode other than
 **Auto** or **Bypass** a task may stop early when Claude Code asks for approval (in Auto, blocked
-actions are denied instead); the run log records the mode
+actions are denied instead, unless Auto is unavailable and Claude Code falls back to prompting); the run log records the mode
 that was used.
 
 ### Creating Tasks