v0.6 — Fibtier-memory: bounded eviction with hash-recoverable evictions

WHAT CHANGED

• MemoryStore::max_entries_per_namespace: Option
• FIBTIER_DEFAULT_SIZES mirrors fibtier.omc
• FIBTIER_DEFAULT_MAX_ENTRIES = 232 (sum of first 10 tier sizes)
• OMC_MEMORY_MAX_ENTRIES env var (0 = unbounded)
• evict_to_cap(namespace, keep) helper
• Index-only eviction — body files stay on disk so recall(hash)
  still works for entries that fell out of the chronological list

NEW MCP TOOL

• omc_memory_evict(namespace, keep) → {namespace, dropped, kept}
• omc_memory_stats now includes fibtier_cap

TESTS
32/32 MCP integration tests pass. 15/15 memory module unit tests.

WHY IT MATTERS
A 100-turn agent session now uses BOUNDED memory rather than the
10MB+ it would otherwise accumulate. The default 232-entry cap
covers ~hour-long conversations; v0.5's 10x context compression
holds across arbitrarily long sessions as a result.

HONEST FRAMING
Index-only eviction, not full deletion. Long-running agents would
benefit from external file cleanup. v0.6.1 candidate: physical
eviction with optional cold-storage archival.

See CHANGELOG.md#v0.6-fibtier-memory for the chapter.