fix: skill invocation during plan mode takes precedence over generic plan mode

Adds a "Skill Invocation During Plan Mode" section to the preamble resolver so
all generated SKILL.md files include it. Fixes a bug where Claude treats loaded
skill content as reference material instead of executable instructions, and keeps
trying to ExitPlanMode instead of following the skill workflow step by step.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

learn/SKILL.md

@@ -423,6 +423,31 @@ artifacts that inform the plan, not code changes:
 These are read-only in spirit — they inspect the live site, generate visual artifacts,
 or get independent opinions. They do NOT modify project source files.
 
+## Skill Invocation During Plan Mode
+
+If a user invokes a skill during plan mode, that invoked skill workflow takes
+precedence over generic plan mode behavior until it finishes or the user explicitly
+cancels that skill.
+
+Treat the loaded skill as executable instructions, not reference material. Follow
+it step by step. Do not summarize, skip, reorder, or shortcut its steps.
+
+If the skill says to use AskUserQuestion, do that. Those AskUserQuestion calls
+satisfy plan mode's requirement to end turns with AskUserQuestion.
+
+If the skill reaches a STOP point, stop immediately at that point, ask the required
+question if any, and wait for the user's response. Do not continue the workflow
+past a STOP point, and do not call ExitPlanMode at that point.
+
+If the skill includes commands marked "PLAN MODE EXCEPTION — ALWAYS RUN," execute
+them. The skill may edit the plan file, and other writes are allowed only if they
+are already permitted by Plan Mode Safe Operations or explicitly marked as a plan
+mode exception.
+
+Only call ExitPlanMode after the active skill workflow is complete and there are no
+other invoked skill workflows left to run, or if the user explicitly tells you to
+cancel the skill or leave plan mode.
+
 ## Plan Status Footer
 
 When you are in plan mode and about to call ExitPlanMode: