feat(mesh): upgrade to mesh-llm 0.76.0-rc8 and recommend Qwen3.8 27B (#6189)

Upgrades Buzz's mesh-llm dependencies through the released `v0.76.0-rc8`
tag (`2040765d`), including the Qwen3.8 curated recommendation and rc8's
scheduler/runtime improvements.

**Scope note:** the earlier open-relay/unenforced-admission mode has
been removed from this PR at Mic's direction — it is not a product mode
we want. Mesh admission remains roster/allowlist driven, exactly as on
`main`: on a relay with no NIP-43 membership snapshot the mesh runs
self-only. No NIP-11 mode probing, no mode-transition restarts. A future
perimeter/admission strategy for open relays will be designed
separately.

This PR also:
- seeds `BUZZ_AGENT_LLM_TIMEOUT_SECS=660` for mesh agents, above
MeshLLM's 600-second backend timeout;
- makes `desktop-tauri-clippy` lint both default and `mesh-llm` cfg
graphs;
- runs the feature-enabled desktop test suite in CI;
- recommends Qwen3.8 27B Q4_K_M for 64 GB-and-larger machines, then
ladders down through Gemma 4 E4B and Qwen 9B for smaller machines;
- keeps stored shared-compute `auto` translated to MeshLLM's supported
wire model `mesh`.

RC8 verification:
- `just ci` passed locally at
`92ecc7ec933bdd4df804cc9f28a2b51efa5313c5`.
- Pre-push differential gates passed, including both desktop Tauri cfg
graphs and package tests.
- A prior isolated runtime smoke used the RC8 binary's OpenAI endpoint
for a Buzz-shaped system/user/tool/tool-result/final-response loop; all
assertions passed and the isolated process was shut down.

Perf previously measured on M5 Metal, Qwen3.8-27B-Q4_K_M: TTFT 0.22–0.32
s, ~25 tok/s streaming; agent-shaped turns ~1 s to first token after the
first (prefix cache).

---------

Signed-off-by: Michael Neale <michael.neale@gmail.com>
Signed-off-by: Jimmy <1fe240cd1a8cf775f6f3060f115e5a303181f3abf28ad4cb0c2515f4a02b36a8@meshllm.communities.buzz.xyz>
Signed-off-by: Alessandro Joabar <sandro@squareup.com>
Co-authored-by: Michael Neale <michael.neale@gmail.com>
Co-authored-by: Jimmy <1fe240cd1a8cf775f6f3060f115e5a303181f3abf28ad4cb0c2515f4a02b36a8@meshllm.communities.buzz.xyz>
Co-authored-by: Mic Neale <305999590+micspiral@users.noreply.github.com>
Co-authored-by: Alessandro Joabar <sandro@squareup.com>

.github/workflows/_ci-desktop.yml

@@ -92,6 +92,10 @@ jobs:
         run: just desktop-tauri-test
         env:
           CMAKE_POLICY_VERSION_MINIMUM: "3.5"
+      - name: Desktop Mesh feature tests
+        run: cargo test --manifest-path desktop/src-tauri/Cargo.toml --features mesh-llm --lib
+        env:
+          CMAKE_POLICY_VERSION_MINIMUM: "3.5"
       - name: Desktop Tauri compiled-flag verification
         run: just desktop-tauri-test-compiled-flags
         env: