fix(workflows): prevent auto_advance config from persisting across sessions

Introduce ephemeral `workflow._auto_chain_active` flag to separate chain
propagation from the user's persistent `workflow.auto_advance` preference.

Previously, `workflow.auto_advance` was set to true by --auto chains and
only cleared at milestone completion. If a chain was interrupted (context
limit, crash, user abort), the flag persisted in .planning/config.json
and caused all subsequent manual invocations to auto-advance unexpectedly.

The fix adds a "sync chain flag with intent" step to discuss-phase,
plan-phase, and execute-phase workflows: when --auto is NOT in arguments,
the ephemeral _auto_chain_active flag is cleared. The persistent
auto_advance setting (from /gsd:settings) is never touched, preserving
the user's deliberate preference.

Closes #857

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

agents/gsd-executor.md

@@ -205,13 +205,14 @@ Do NOT continue reading. Analysis without action is a stuck signal.
 </authentication_gates>
 
 <auto_mode_detection>
-Check if auto mode is active at executor start:
+Check if auto mode is active at executor start (chain flag or user preference):
 
 ```bash
+AUTO_CHAIN=$(node "$HOME/.claude/get-shit-done/bin/gsd-tools.cjs" config-get workflow._auto_chain_active 2>/dev/null || echo "false")
 AUTO_CFG=$(node "$HOME/.claude/get-shit-done/bin/gsd-tools.cjs" config-get workflow.auto_advance 2>/dev/null || echo "false")
 ```
 
-Store the result for checkpoint handling below.
+Auto mode is active if either `AUTO_CHAIN` or `AUTO_CFG` is `"true"`. Store the result for checkpoint handling below.
 </auto_mode_detection>
 
 <checkpoint_protocol>