Upgrade dependencies and toolchain; migrate to Claude API 2026-10-21

Toolchain and CI:
- Go 1.27 (go directive 1.27.0, CI 1.27.3)
- actions/checkout v7, actions/setup-go v7
- golangci-lint v2.15.0, gosec v2.30.0
- Dockerfile: drop obsolete -a -installsuffix cgo build flags

Go modules:
- anthropic-sdk-go v1.71.0 -> v2.3.0 (new /v2 module path, API version
  2026-10-21); go-telegram/bot v1.27.0; x/sync v0.24.0; indirects to latest
- BetaMessageParam is now a per-role union: build assistant turns with
  OfAssistant and read content through the role in the cache helper
- Pass the Anthropic API key only when set: v2 sends an explicitly passed
  credential as is, so an empty key became an empty bearer token instead of
  falling back to the SDK's default credential chain as v1 did

API 2026-10-21 / deprecations:
- Stop sending temperature (rejected on every model); the config key is
  still parsed and a startup notice flags it
- Name refusals in the empty-response error instead of reporting an
  unexpected response format
- ElevenLabs STT: scribe_v1 (deprecated) -> scribe_v2
- Template config: claude-haiku-5-5 with effort low, temperature removed

Docs: replace the unsupported `go get -u` install step, document the ignored
temperature key, default-on thinking, and per-model effort defaults.

config/default.json

@@ -11,9 +11,9 @@
     "messages_per_hour": 20,
     "messages_per_day": 100,
     "temp_ban_duration": "24h",
-    "model": "claude-haiku-4-5",
-    "temperature": 0.7,
+    "model": "claude-haiku-5-5",
     "max_tokens": 1000,
+    "effort": "low",
     "debounce_ms": 2500,
     "debug_screening": false,
     "system_prompts": {