feat: avoid duplicate project-map hint injection by checking context

The before_agent_start handler now inspects the current session context
and skips injection if a pi-project-map-hint custom message is already
present in the active branch. This prevents duplicate hints on every
prompt while still re-injecting after compaction or tree navigation.

Also removes the unused hooks/on-prompt.ts prompt-text injector.

tests/pi-extension.test.ts

@@ -48,6 +48,9 @@ describe("pi-extension", () => {
 				getApiKeyAndHeaders: vi.fn(async () => ({ apiKey: "test-key", headers: {} })),
 			},
 			ui: { notify: mockNotify },
+			sessionManager: {
+				buildSessionContext: vi.fn(() => ({ messages: [] })),
+			},
 		};
 
 		const mockPi = {
@@ -225,5 +228,44 @@ describe("pi-extension", () => {
 
 			expect(result).toEqual({});
 		});
+
+		it("does not inject hint when it is already in context", async () => {
+			const dir = mkdtempSync(join(tmpdir(), "pi-ext-test-"));
+			writeFileSync(join(dir, ".pi-map.md"), "# .\n## role\nTest\n");
+			mockCtx.cwd = dir;
+			mockCtx.sessionManager.buildSessionContext = vi.fn(() => ({
+				messages: [
+					{
+						role: "custom",
+						customType: "pi-project-map-hint",
+						content: "existing hint",
+						display: false,
+					},
+				],
+			}));
+
+			const handler = registeredEvents.before_agent_start;
+			const result = await handler(null, mockCtx);
+
+			expect(result).toEqual({});
+		});
+
+		it("injects hint again when previous context did not contain it", async () => {
+			const dir = mkdtempSync(join(tmpdir(), "pi-ext-test-"));
+			writeFileSync(join(dir, ".pi-map.md"), "# .\n## role\nTest\n");
+			mockCtx.cwd = dir;
+			mockCtx.sessionManager.buildSessionContext = vi.fn(() => ({
+				messages: [
+					{ role: "user", content: "hello" },
+					{ role: "assistant", content: [{ type: "text", text: "hi" }] },
+				],
+			}));
+
+			const handler = registeredEvents.before_agent_start;
+			const result = await handler(null, mockCtx);
+
+			expect(result).toHaveProperty("message");
+			expect(result.message.customType).toBe("pi-project-map-hint");
+		});
 	});
 });