fix: use per-chat plan file paths (#24268)

> This PR was authored by Mux on behalf of Mike.

Chats sharing one workspace (e.g. sibling subagents) all wrote to
`/home/coder/PLAN.md`, causing plan file collisions. This change derives
a unique plan path per chat from the workspace home directory and chat
ID.

## Changes

* `write_file`, `edit_files`, and `propose_plan` reject any `plan.md`
variant (case-insensitive) at the workspace home root, with a clear
error pointing to the chat-specific path.
* Root chats receive a `<plan-file-path>` block inlined in the main
system prompt with the concrete path.
* Prompt and tool descriptions no longer hardcode `/home/coder/PLAN.md`.
* Plan path handling is POSIX-only (forward-slash), relying on the
contract that workspace agent paths are normalized before reaching
chatd.
* Updated `ProposePlanTool.stories.tsx` to use per-chat path examples.
* Full test coverage for plan path detection, legacy-path rejection in
all three tools, inline prompt rendering, and fallback behavior.

coderd/x/chatd/chattool/planpathmessage.go

@@ -0,0 +1,54 @@
+package chattool
+
+import (
+	"fmt"
+
+	"charm.land/fantasy"
+)
+
+// rejectSharedPlanPath reports whether requestedPath targets the shared
+// home-root plan file and, if so, returns a rejection response that
+// points callers at the chat-specific plan path.
+func rejectSharedPlanPath(
+	requestedPath string,
+	home string,
+	chatPath string,
+	planPathErr error,
+) (fantasy.ToolResponse, bool) {
+	if planPathErr != nil {
+		// When the resolver fails, we cannot determine the actual
+		// home directory. Fall back to rejecting only the exact
+		// legacy shared path (case-insensitive) rather than every
+		// file named plan.md.
+		if !looksLikeLegacySharedPlanPath(requestedPath) {
+			return fantasy.ToolResponse{}, false
+		}
+
+		return fantasy.NewTextErrorResponse(
+			planPathVerificationMessage(requestedPath),
+		), true
+	}
+
+	if !LooksLikeHomePlanFile(requestedPath, home) && !looksLikeLegacySharedPlanPath(requestedPath) {
+		return fantasy.ToolResponse{}, false
+	}
+
+	return fantasy.NewTextErrorResponse(
+		sharedPlanPathMessage(requestedPath, chatPath),
+	), true
+}
+
+func sharedPlanPathMessage(requestedPath, chatPath string) string {
+	return fmt.Sprintf(
+		"the plan path %s is no longer supported at the home root; use the chat-specific plan path: %s",
+		requestedPath,
+		chatPath,
+	)
+}
+
+func planPathVerificationMessage(requestedPath string) string {
+	return fmt.Sprintf(
+		"the plan path %s could not be verified because the workspace is currently unavailable to resolve the chat-specific plan path, try again shortly",
+		requestedPath,
+	)
+}