feat: add propose_plan tool for markdown plan proposals (#23452)

Adds a `propose_plan` tool that presents a workspace markdown file as a
dedicated plan card in the agent UI.

The workflow is: the agent uses `write_file`/`edit_files` to build a
plan file (e.g. `/home/coder/PLAN.md`), then calls `propose_plan(path)`
to present it. The backend reads the file via `ReadFile` and the
frontend renders it as an expanded markdown preview card.

**Backend** (`coderd/x/chatd/chattool/proposeplan.go`): new tool
registered as root-chat-only. Validates `.md` suffix, requires an
absolute path, reads raw file content from the workspace agent. Includes
1 MiB size cap.

**Frontend** (`site/src/components/ai-elements/tool/`): dedicated
`ProposePlanTool` component with `ToolCollapsible` + `ScrollArea` +
`Response` markdown renderer, expanded by default. Custom icon
(`ClipboardListIcon`) and filename-based label.

**System prompt** (`coderd/x/chatd/prompt.go`): added `<planning>`
section guiding the agent to research → write plan file → iterate → call
`propose_plan`.

coderd/x/chatd/chatd_test.go

@@ -218,7 +218,7 @@ func TestSubagentChatExcludesWorkspaceProvisioningTools(t *testing.T) {
 	require.GreaterOrEqual(t, len(recorded), 2,
 		"expected at least 2 streamed LLM calls (root + subagent)")
 
-	workspaceTools := []string{"list_templates", "read_template", "create_workspace"}
+	workspaceTools := []string{"propose_plan", "list_templates", "read_template", "create_workspace"}
 	subagentTools := []string{"spawn_agent", "wait_agent", "message_agent", "close_agent"}
 
 	// Identify root and subagent calls. Root chat calls include