fix(compaction): route plan-mode guidance via internalGuidance channel

Plan-approval's 'Approve and compact context' used to pass the rendered
plan-mode-compact-instructions prompt as the first positional argument
to handleCompactCommand -> session.compact(), which landed on the
session_before_compact extension hook as customInstructions. Extensions
treating that field as user focus (e.g. to bias a query-focused summary)
would then see plan-mode boilerplate instead of operator intent and
produce query-biased compactions.

Add CompactOptions.internalGuidance: a private summarizer-only channel.
session.compact() reads it into the fallback-model summarizer while the
session_before_compact hook payload still only carries the public
customInstructions arg (undefined for the plan-compact path). The
snapcompact-disable predicate and the /compact rejectsFocus guard cover
both fields so a directed summary is never silently downgraded.

Extend the interactive-mode handleCompactCommand facade + command
controller with a fourth internalGuidance parameter, and switch the
plan-approval callsite in interactive-mode.ts to route the plan prompt
through it.

Fixes #4359

packages/coding-agent/src/modes/controllers/command-controller.ts

@@ -1124,6 +1124,7 @@ export class CommandController {
 		customInstructions?: string,
 		mode?: CompactMode,
 		beforeFlush?: (outcome: CompactionOutcome) => void | Promise<void>,
+		internalGuidance?: string,
 	): Promise<CompactionOutcome> {
 		const entries = this.ctx.sessionManager.getEntries();
 		const messageCount = entries.filter(e => e.type === "message").length;
@@ -1133,6 +1134,15 @@ export class CommandController {
 			return "ok";
 		}
 
+		// `internalGuidance` is a private summarizer directive (plan-mode
+		// "Approve and compact context") that MUST stay off the public
+		// `customInstructions` channel of the `session_before_compact` extension
+		// hook — extensions treat that field as user focus and would otherwise
+		// bias the summary toward the plan boilerplate (issue #4359). Ride it
+		// through as a CompactOptions field instead.
+		if (internalGuidance) {
+			return this.executeCompaction({ internalGuidance, ...(mode ? { mode } : {}) }, false, beforeFlush, mode);
+		}
 		return this.executeCompaction(customInstructions, false, beforeFlush, mode);
 	}