fix(agent-runtime): never wipe chat history on compaction

[nordicnode]

Summary

Two verified paths could replace the model's entire context with nothing (issue #1166, from the "Model losing all chat history instead of compacting" reports):

• /compact with an empty model response — the replacement summary was built from fullResponse without an emptiness check, and a silent empty stop yields no recovery chunk, so the whole history was replaced by a single message carrying nothing. Now the replacement only happens when the model actually produced a summary; otherwise the history is kept (the forced next step retries) and a warning is logged.
• Summary identity by content — a user message quoting the summary markers matched isConversationSummary, so findLast picked the quote over the real summary and the real summary was neither re-parsed nor kept: every earlier turn vanished at the next compaction. Summaries are now stamped with a CONVERSATION_SUMMARY tag and recognized by provenance; a legacy fallback requires the full envelope (open+close tag, header, <historical_memory>) so pre-tag summaries still fold in. The context-pruner's inlined copy — which matched the bare tag alone, wiping memory on any message that merely mentioned the tag — gets the same treatment, closing the parity test's second deliberate divergence.

Regression tests for both mechanisms, including the exact steal shape (tag + header quote with no memory block) and the legacy-envelope case. The parity test now asserts BOTH implementations keep a tag-mentioning user message.

Validation

• bun test packages/agent-runtime/src/__tests__/compact-history.test.ts packages/agent-runtime/src/__tests__/context-pruner-parity.test.ts packages/agent-runtime/src/__tests__/main-prompt.test.ts (55 pass, 7 new/updated)
• Verified the new /compact test fails without the guard by stashing run-agent-step.ts
• bun test packages/agent-runtime (only the 2 pre-existing prompts-schema-handling failures and 2 pre-existing agents-graveyard module-not-found errors remain — same on main)
• bunx tsc --noEmit -p agents/tsconfig.json clean

Fixes #1166