feat: add plan mode with restricted tool boundary (#24236)

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

## Summary
- add persistent plan mode for chats and the chat-specific plan file
flow
- add structured planning tools such as `ask_user_question` and
`propose_plan`
- keep `write_file` and `edit_files` constrained to the chat-specific
plan file during plan turns
- allow shell exploration in plan mode, including subagents, via
`execute` and `process_output`
- block implementation-oriented, provider-native, MCP, dynamic, and
computer-use tools during plan turns
- update the chat UI, tests, and docs for the new planning flow

coderd/database/db2sdk/db2sdk.go

@@ -1601,6 +1601,9 @@ func Chat(c database.Chat, diffStatus *database.ChatDiffStatus, files []database
 	if c.LastError.Valid {
 		chat.LastError = &c.LastError.String
 	}
+	if c.PlanMode.Valid {
+		chat.PlanMode = codersdk.ChatPlanMode(c.PlanMode.ChatPlanMode)
+	}
 	if c.ParentChatID.Valid {
 		parentChatID := c.ParentChatID.UUID
 		chat.ParentChatID = &parentChatID

coderd/database/db2sdk/db2sdk_test.go

@@ -817,6 +817,7 @@ func TestChat_AllFieldsPopulated(t *testing.T) {
 		UpdatedAt:         now,
 		Archived:          true,
 		PinOrder:          1,
+		PlanMode:          database.NullChatPlanMode{ChatPlanMode: database.ChatPlanModePlan, Valid: true},
 		MCPServerIDs:      []uuid.UUID{uuid.New()},
 		Labels:            database.StringMap{"env": "prod"},
 		LastInjectedContext: pqtype.NullRawMessage{