fix(cache): kill long-lived prefix layout — system prompt is now byte-static within a session (#24778)

The long-lived prefix-cache layout split the system prompt into stable/
context/volatile blocks and re-derived them on every API call. The
volatile tier (timestamp + memory snapshot + USER profile) ticks per
turn, so the system message bytes mutated mid-conversation and broke
upstream prompt caches (OpenRouter, Nous Portal, Anthropic).

Diagnosed via live wire-format diffing: an 8-turn conversation showed
OLD layout flipping system block[1] sha mid-session at the minute
boundary, dropping cached_tokens to 0 on that turn (cumulative
66.6% vs 83.3% for the single-block layout). Hermes invariant:
history (system + all but the last 1-2 messages) must be static.

Fix: drop the long-lived layout entirely. Single layout everywhere —
system_and_3 with one cached system string built once on first turn,
replayed verbatim on every subsequent turn. Loses cross-session 1h
prefix caching for Claude (the feature that motivated the split), but
within-session caching now actually works on every provider.

Removed:
- run_agent.py: _use_long_lived_prefix_cache flag, _long_lived_cache_ttl,
  _supports_long_lived_anthropic_cache method, the long-lived branch in
  run_conversation, mark_tools_for_long_lived_cache call site
- agent/prompt_caching.py: apply_anthropic_cache_control_long_lived,
  mark_tools_for_long_lived_cache, _mark_system_stable_block helper
- hermes_cli/config.py: prompt_caching.long_lived_prefix and
  prompt_caching.long_lived_ttl config keys
- tests/agent/test_prompt_caching_live.py (entire file)
- tests/agent/test_prompt_caching.py: TestMarkToolsForLongLivedCache,
  TestApplyAnthropicCacheControlLongLived
- tests/run_agent/test_anthropic_prompt_cache_policy.py:
  TestSupportsLongLivedAnthropicCache

Targeted tests: 62/62 pass.

hermes_cli/config.py

@@ -735,15 +735,8 @@ DEFAULT_CONFIG = {
 
     # Anthropic prompt caching (Claude via OpenRouter or native Anthropic API).
     # cache_ttl must be "5m" or "1h" (Anthropic-supported tiers); other values are ignored.
-    # long_lived_prefix: when true (default), Claude on Anthropic / OpenRouter / Nous
-    #   Portal uses a split layout: tools[-1] + stable system prefix at long_lived_ttl
-    #   (cross-session cache), last 2 messages at cache_ttl (within-session rolling).
-    #   Set false to keep the legacy "system + last 3 messages" single-tier layout.
-    # long_lived_ttl: TTL for the cross-session prefix tier ("5m" or "1h"; default "1h").
     "prompt_caching": {
         "cache_ttl": "5m",
-        "long_lived_prefix": True,
-        "long_lived_ttl": "1h",
     },
 
     # OpenRouter-specific settings.