feat(#20): worldtree-sdk cutover teardown (slice-7) + v0.22.0

The last slice of the consumer-layer cutover. Teardown only — zero
runtime-logic change; the 494-green suite is the regression gate.

- Drop `httpx-sse` from pyproject + lockfile: slice-6 deleted its last
  user, nothing imports `httpx_sse`, the SDK owns SSE parsing now.
- Module boundary (operator decision): KEEP `sessions.py` + `sse_client.py`
  as pure caller-semantic type/exception homes (no rename, no fold —
  A3 was blocked by the `AgentNotAvailable` name collision + `wt.py`
  would mis-home `endpoint_for_plane`). Docstrings updated to stop
  claiming "client"; the `AdminEvent`/`SseId`/exception homes stay put
  (resolves the slice-6 deferred-home item).
- Retire wire contracts #2 (sessions) + #15 (tier3): DEC-1 phase-2 —
  normative authority already transferred to the cutover contract; the
  code they specified is gone, so the files are deleted. #1 (SSE event
  vocab) and `first_message` stay (ratatoskr-owned, not retired).
- Final coverage-map re-anchor: tools/list_sessions re-homed to `wt.py`;
  the Last-Event-ID SSE-resume sub-gap CLOSED (folded into `stream_turn`
  auto-resume); Surface-2 SSE parsing re-anchored to the SDK.
- Stale doc-rot fix: the cli.py transport comment no longer calls
  `seed_preset_first_message` "not-yet-migrated" (it rides `wt`).
- v0.22.0 (minor, DEC-6, operator-approved): publishes the full 6-slice
  cutover milestone.

uv.lock

@@ -183,15 +183,6 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/2a/39/e50c7c3a983047577ee07d2a9e53faf5a69493943ec3f6a384bdc792deb2/httpx-0.28.1-py3-none-any.whl", hash = "sha256:d909fcccc110f8c7faf814ca82a9a4d816bc5a6dbfea25d6591d6985b8ba59ad", size = 73517 },
 ]
 
-[[package]]
-name = "httpx-sse"
-version = "0.4.3"
-source = { registry = "https://pypi.org/simple" }
-sdist = { url = "https://files.pythonhosted.org/packages/0f/4c/751061ffa58615a32c31b2d82e8482be8dd4a89154f003147acee90f2be9/httpx_sse-0.4.3.tar.gz", hash = "sha256:9b1ed0127459a66014aec3c56bebd93da3c1bc8bb6618c8082039a44889a755d", size = 15943 }
-wheels = [
-    { url = "https://files.pythonhosted.org/packages/d2/fd/6668e5aec43ab844de6fc74927e155a3b37bf40d7c3790e49fc0406b6578/httpx_sse-0.4.3-py3-none-any.whl", hash = "sha256:0ac1c9fe3c0afad2e0ebb25a934a59f4c7823b60792691f779fad2c5568830fc", size = 8960 },
-]
-
 [[package]]
 name = "idna"
 version = "3.15"
@@ -472,11 +463,10 @@ wheels = [
 
 [[package]]
 name = "ratatoskr"
-version = "0.21.20"
+version = "0.22.0"
 source = { editable = "." }
 dependencies = [
     { name = "httpx" },
-    { name = "httpx-sse" },
     { name = "worldtree-sdk" },
 ]
 
@@ -507,7 +497,6 @@ web = [
 requires-dist = [
     { name = "bifrost", marker = "extra == 'provider'", specifier = "==1.1.4", index = "https://gitea.phasefinal.com/api/packages/vh/pypi/simple/" },
     { name = "httpx", specifier = ">=0.27" },
-    { name = "httpx-sse", specifier = ">=0.4" },
     { name = "jsonschema", marker = "extra == 'provider'", specifier = ">=4" },
     { name = "mypy", marker = "extra == 'dev'", specifier = ">=1.11" },
     { name = "pytest", marker = "extra == 'dev'", specifier = ">=8" },