refactor: no feature depends on Gemini any more

Depleted Gemini prepayment credits took out PDF rendering. The same key was the
only thing standing between level-up proposals and the same fate, so both are
off it.

- `pdf_renderer` is DELETED, not disabled. Nothing sets `render_pdf: true` since
  markdown became the deliverable (821cbb8), so the worker polled forever for
  rows that can no longer exist. It was also the only caller of the Gemini
  MD->HTML conversion. A worker that cannot do anything is worse than absent: it
  reads as a feature.

- `level_up` now resolves its proposer through the provider REGISTRY
  (`Runtime::resolve_provider`), the same path the evaluator uses, defaulting to
  `glm:glm-4.7` — the validator this project measured and chose in
  scripts/judge-eval.sh. `CLAWMATES_LEVEL_UP_MODEL` takes a registry spec
  (`glm:glm-4.7`, `kimi:k2`, `claude-sonnet-5`), so every provider the platform
  can already reach works and no single vendor's billing can take it down.

The non-obvious part of that swap: Gemini was asked for
`response_mime_type: application/json` and obliged, so the old code parsed the
raw reply. Anthropic-format models are under no such obligation and wrap objects
in prose or a ```json fence. `extract_json_object` brace-counts to the matching
close — string-aware, so a `}` inside a value does not end it, and nested (these
proposals nest by design). Tested against bare, fenced, nested, brace-in-string
and absent. Parsing raw text would have worked in review and failed on the first
real proposal.

What deliberately still MENTIONS Gemini: `mission_runtime` forwards
GEMINI_API_KEY to agent containers alongside GROQ/OPENAI/ZAI/KIMI, and the claw
model selector offers it. Those are user options, not platform requirements —
the ask was to remove the NEED.

Also corrected a comment in mission_delivery that cited `pdf_renderer` as the
authority on artifact path resolution. It never was: it joined the mission id
first and produced a doubled path that never resolved.

238 lib tests, 20 test binaries.

crates/bins/clawmates-server/src/main.rs

@@ -346,12 +346,6 @@ async fn run() -> Result<(), String> {
     // asks Claude Opus 4.8 to synthesize a "what got done" card that
     // the UI renders under the phase.
     cm_api::phase_summarizer::spawn(pool.clone());
-    // PDF renderer worker (Slice 6): watches mission_artifacts for
-    // MD entries with render_pdf_status='pending', calls the
-    // configured LLM (default Gemini 2.5 Flash) for styled HTML,
-    // prints to PDF via chromium --headless. No-op-friendly when
-    // GEMINI_API_KEY / chromium binary aren't configured.
-    cm_api::pdf_renderer::spawn(pool.clone());
     // Outbound-email delivery: drains the §15-gated `outbox` over SMTP. Inert
     // until CLAWMATES_SMTP_* is set, so it ships safely before credentials exist.
     cm_runtime::spawn_drainer(pool.clone(), std::time::Duration::from_secs(10));