feat: opencode/tinfoil backend support in all launchers

Adds LOOP_BACKEND=opencode|claude (+ LOOP_MODEL) to launch.sh and
launch-upgrader.sh, enabling the loops/upgrader to run via opencode CLI
against the tinfoil.sh API (deepseek-v4-pro etc.) instead of Claude.

launch.sh:
- LOOP_BACKEND (claude|opencode), LOOP_MODEL env vars
- OPENCODE_BIN, OPENCODE_HOST (tailscale IP), OPENCODE_PORT (per-session)
- start_agent: backend switch — claude path unchanged; opencode starts
  `opencode --hostname <ts-ip> --port <N> run <kickoff>` so the web UI
  is bound to the tailscale interface (tailnet-only observability)
- preflight: validates the right binary per backend
- heal_session / heal_orchestrator: extend active-work detection to
  opencode spinner chars + "Running tool"
- help: shows both backend configs

launch-upgrader.sh:
- UPGRADER_BACKEND / UPGRADER_MODEL (LOOP_BACKEND/LOOP_MODEL override)
- start: same backend switch as launch.sh
- OPENCODE_PORT=4098 (separate from loops 4096/4097)

configuration.nix: note opencode binary location + re-install command.

Tinfoil config: ~/.config/opencode/opencode.jsonc — provider "tinfoil"
with baseURL=https://api.tinfoil.sh/v1, apiKey=env:TINFOIL_API_KEY
(key + TINFOIL_MODEL + TINFOIL_BASE_URL stored in .testenv).
opencode v1.15.13 installed at /home/loops/.local/bin/opencode.

Usage:
  LOOP_BACKEND=opencode LOOP_MODEL=tinfoil/deepseek-v4-pro \
    RESUME_PHASE=1 cc-ci-plan/launch.sh start

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nix/hosts/cc-ci-orchestrator-hetzner/configuration.nix

@@ -70,7 +70,10 @@
     commands = [{ command = "ALL"; options = [ "NOPASSWD" ]; }];
   }];
 
-  # Ensure /home/loops/.local/bin (claude) is on the loops user PATH.
+  # Ensure /home/loops/.local/bin (claude + opencode) is on the loops user PATH.
+  # opencode binary is installed there manually (not yet in nixpkgs); re-install if missing:
+  #   curl -sL https://github.com/anomalyco/opencode/releases/download/v1.15.13/opencode-linux-x64.tar.gz \
+  #     | tar -xz -C /home/loops/.local/bin opencode && chmod +x /home/loops/.local/bin/opencode
   environment.variables.PATH = lib.mkForce
     "/home/loops/.local/bin:/run/current-system/sw/bin:/run/wrappers/bin:/usr/bin:/bin";