perf(agent): keep the prompt-cache prefix stable across harness updates, date flips, and tool refreshes

[rynfar]

Part of #22 (Meridian reliability umbrella). P1.

Problem

Anthropic prompt caching requires a byte-stable prefix (system prompt + tool definitions + message history). Prime Agent's system prompt embeds volatile state, so unrelated actions invalidate the whole cache for a long-running session:

• _rebuildSystemPrompt (packages/coding-agent/src/core/agent-session.ts) serializes harnessState: this._loadMergedHarnessState() — mutable continual-harness state (memories, skills, subagent specs) — directly into the system prompt. Any rlm.harness.create_* call by the model changes the cached prefix for every subsequent request.
• Current date: ${date} (packages/coding-agent/src/core/system-prompt.ts) flips the prefix daily — long-running/scheduled sessions cold-cache at midnight.
• Tool-set changes (_refreshToolRegistry) reorder/replace context.tools, and cache_control is placed on the last tool definition, so any reorder invalidates the tool-block cache.
• The extension context hook (extensions/runner.ts emitContext) lets any extension rebuild the message array per call — a silent prefix-breaker worth a debug-level stability check.

Through Meridian this compounds: a prefix change on one turn forces the Claude Code SDK session to re-cache the full conversation (5-min-TTL cache), which is exactly the "expensive cache churn" measured in #22 (65.8% avg hit rate vs 95.1% for OpenCode).

Proposed shape

• Move volatile content (harness state, current date) out of the cached prefix — e.g. into a trailing uncached system block or the first user message — so refining the harness doesn't cold-cache the session.
• Stabilize tool ordering (stable sort by name) so the cache_control-marked last tool doesn't move when the registry refreshes.
• Optional: a debug assertion that logs when a rebuilt system prompt/tool block differs from the previous turn's, with a diff summary — makes future prefix regressions visible instead of silently expensive.

Acceptance

• A rlm.harness.create_memory call mid-session does not change the system-prompt bytes of the next request's cached prefix region (test at the buildParams/payload seam).
• Tool registry refresh with an unchanged tool set produces byte-identical tool definitions in the payload.