fix(security): bump vulnerable deps; harden git_add (defense-in-depth)

Real fixes (deps):
- python (git/fetch/time): idna 3.10->3.18 (GHSA-65pc), starlette
  0.49.1->1.3.1 (GHSA-86qp)
- npm: vitest ^2.1.8->^4.1.8 (critical GHSA-5xrq UI server RCE). vitest 4
  pulls vite 8 (rolldown, no esbuild), clearing the esbuild/vite advisories
  natively. Remaining root overrides: qs>=6.15.2 (GHSA-q8mj, via express),
  hono>=4.12.21 (via @modelcontextprotocol/sdk).
- npm audit: 0 vulnerabilities; all TS (313) + Python (git/fetch/time) suites
  and TS builds pass.

Hardening (git_add):
- The repo already mitigates CVE-2026-27735 by using repo.git.add (the git
  CLI rejects out-of-tree paths, PR #3164's fix), not repo.index.add (which
  did not). Add an explicit path-boundary check in git_add as defense in
  depth plus regression tests asserting the security invariant (out-of-tree
  paths are never staged), guarding against a regression to repo.index.add.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/fetch/uv.lock

@@ -360,11 +360,11 @@ wheels = [
 
 [[package]]
 name = "idna"
-version = "3.10"
+version = "3.18"
 source = { registry = "https://pypi.org/simple" }
-sdist = { url = "https://files.pythonhosted.org/packages/f1/70/7703c29685631f5a7590aa73f1f1d3fa9a380e654b86af429e0934a32f7d/idna-3.10.tar.gz", hash = "sha256:12f65c9b470abda6dc35cf8e63cc574b1c52b11df2c86030af0ac09b01b13ea9", size = 190490, upload-time = "2024-09-15T18:07:39.745Z" }
+sdist = { url = "https://files.pythonhosted.org/packages/cd/63/9496c57188a2ee585e0f1db071d75089a11e98aa86eb99d9d7618fc1edce/idna-3.18.tar.gz", hash = "sha256:ffb385a7e039654cef1ab9ef32c6fafe283c0c0467bba1d9029738ce4a14a848", size = 196711, upload-time = "2026-06-02T14:34:07.794Z" }
 wheels = [
-    { url = "https://files.pythonhosted.org/packages/76/c6/c88e154df9c4e1a2a66ccf0005a88dfb2650c1dffb6f5ce603dfbd452ce3/idna-3.10-py3-none-any.whl", hash = "sha256:946d195a0d259cbba61165e88e65941f16e9b36ea6ddb97f00452bae8b1287d3", size = 70442, upload-time = "2024-09-15T18:07:37.964Z" },
+    { url = "https://files.pythonhosted.org/packages/1e/5e/d4e9f1a599fb8e573b7b87160658329fbf28d19eac2718f51fc3def3aa5a/idna-3.18-py3-none-any.whl", hash = "sha256:7f952cbe720b688055e3f87de14f5c3e5fdaa8bc3928985c4077ca689de849a2", size = 65455, upload-time = "2026-06-02T14:34:06.319Z" },
 ]
 
 [[package]]
@@ -1205,15 +1205,15 @@ wheels = [
 
 [[package]]
 name = "starlette"
-version = "0.49.1"
+version = "1.3.1"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
     { name = "anyio" },
     { name = "typing-extensions", marker = "python_full_version < '3.13'" },
 ]
-sdist = { url = "https://files.pythonhosted.org/packages/1b/3f/507c21db33b66fb027a332f2cb3abbbe924cc3a79ced12f01ed8645955c9/starlette-0.49.1.tar.gz", hash = "sha256:481a43b71e24ed8c43b11ea02f5353d77840e01480881b8cb5a26b8cae64a8cb", size = 2654703, upload-time = "2025-10-28T17:34:10.928Z" }
+sdist = { url = "https://files.pythonhosted.org/packages/eb/e3/7c1dc7381d9f8ab7d854328ebfa884e62cb3f3d8549ddfd37c7814f42afa/starlette-1.3.1.tar.gz", hash = "sha256:05d0213193f2fbaae60e2ecb593b4add4262ad4e46536b54abe36f11a71724e0", size = 2703240, upload-time = "2026-06-12T09:23:11.602Z" }
 wheels = [
-    { url = "https://files.pythonhosted.org/packages/51/da/545b75d420bb23b5d494b0517757b351963e974e79933f01e05c929f20a6/starlette-0.49.1-py3-none-any.whl", hash = "sha256:d92ce9f07e4a3caa3ac13a79523bd18e3bc0042bb8ff2d759a8e7dd0e1859875", size = 74175, upload-time = "2025-10-28T17:34:09.13Z" },
+    { url = "https://files.pythonhosted.org/packages/ec/bb/2799cc2ede3ed41131f8975621e7213dfc7ef4acbbaadfa440f32500c370/starlette-1.3.1-py3-none-any.whl", hash = "sha256:c7372aae11c3c3f26a42df7bd626cec2f47d03483d261d369516a615a53714c6", size = 73632, upload-time = "2026-06-12T09:23:10.017Z" },
 ]
 
 [[package]]