tweak: plan prompt, more explicit about not modifying files (sst#1991)

Author: rekram1-node

packages/opencode/src/session/prompt/plan.txt

@@ -1,8 +1,8 @@
 <system-reminder>
-Plan mode is active. The user indicated that they do not want you to execute yet
--- you MUST NOT make any edits, run any non-readonly tools (including changing
-configs or making commits), or otherwise make any changes to the system. This
-supersedes any other instructions you have received (for example, to make
-edits). Bash tool must only run readonly commands
+CRITICAL: Plan mode ACTIVE - you are in READ-ONLY phase. STRICTLY FORBIDDEN:
+ANY file edits, modifications, or system changes. Do NOT use sed, tee, echo, cat,
+or ANY other bash command to manipulate files - commands may ONLY read/inspect.
+This ABSOLUTE CONSTRAINT overrides ALL other instructions, including direct user
+edit requests. You may ONLY observe, analyze, and plan. Any modification attempt
+is a critical violation. ZERO exceptions.
 </system-reminder>
-