Summary

Implements ACON-style adaptive compression guidelines that learn from post-compaction context loss (closes #1647).

• SQLite store (migration 032): compression_guidelines (singleton guidelines doc) and compression_failure_pairs (context/failure pairs with cleanup policy)
• FailureDetector: conservative two-signal heuristic — both an uncertainty phrase AND a prior-context reference must appear; gated entirely behind the feature flag
• GuidelinesUpdater: background tokio task with CancellationToken, 30s LLM timeout, exponential backoff (max 1h), pair cleanup after successful updates
• Guidelines injection: sanitized against prompt-injection patterns before insertion into apply_deferred_summaries and chunked_compaction prompts
• Config: [memory.compression_guidelines] with enabled (default false), update_threshold (5), max_guidelines_tokens (500), max_stored_pairs (100)
• --init wizard: yes/no prompt for enabling compression guidelines
• --migrate-config: default section added to config/default.toml
• Feature flag: compression-guidelines (optional, included in full)

All 2 critical and 4 high findings from design review addressed. Follow-up issues: #1799, #1801–#1803, #1805–#1807.

Test plan

• cargo nextest run --workspace --features full --lib --bins — 5787 passed
• cargo clippy --workspace --features full -- -D warnings — clean
• cargo +nightly fmt --check — clean
• Default build (cargo build) compiles without compression-guidelines feature
• cargo build --features full,compression-guidelines compiles with feature enabled
• Enable in config and run agent through a compaction scenario to verify guidelines injection