docs(changelog): zombie memory-loss counters — six observability families

[indykish]

Adds the Jun 12, 2026 <Update> for the memory-loss observability release (usezombie PR #397, merged): six global zombie_memory_* Prometheus counter families on GET /metrics — hydration-window drops (entries + bytes), cap evictions, capture truncations, capture skips, and zero-hit searches — with the zero-hit integrity guarantee (database errors never count as recall misses) and a note that the three existing memory families are unchanged.

Voice per CHANGELOG_VOICE.md: one headline, consequence-first bullets, family names verbatim, no milestone IDs in the body.

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Greptile Summary

This PR adds a single <Update> block to changelog.mdx documenting the Jun 12, 2026 memory-loss observability release: six new global, label-free zombie_memory_* Prometheus counter families on GET /metrics.

• Six counter families are described (hydration window drops, cap evictions, capture truncations, capture skips, zero-hit searches) with the zero-hit integrity guarantee explained clearly.
• Prose consistently uses "agent"/"per-agent" throughout; all zombie_memory_* identifiers are correctly backtick-wrapped as code tokens per the AGENTS.md terminology rules.

Confidence Score: 5/5

Documentation-only change adding a single changelog entry; no code, schema, or configuration is touched.

The change is a single Update block in a Mintlify MDX changelog. Terminology rules are followed correctly — agent/per-agent in all prose, metric names backtick-wrapped. Section order, voice, and tag conventions match the surrounding entries. No functional code is modified.

No files require special attention.

Important Files Changed

Filename	Overview
changelog.mdx	Adds a Jun 12, 2026 <Update> block documenting six new zombie_memory_* Prometheus counter families; prose correctly uses "agent"/"per-agent" throughout, all metric names are backtick-wrapped code tokens, section order and voice rules are followed.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Agent Memory Event] --> B{Event Type}
    B -->|Hydration reply byte-budget exceeded| C[zombie_memory_hydration_dropped_entries_total\nzombie_memory_hydration_dropped_bytes_total]
    B -->|Per-agent cap hit after capture push| D[zombie_memory_cap_evictions_total]
    B -->|Push byte-budget reached early| E[zombie_memory_capture_truncated_total]
    B -->|Delta rejected by validation| F[zombie_memory_capture_skipped_total]
    B -->|Memory search returns empty result| G{DB error?}
    G -->|No — clean empty result| H[zombie_memory_search_zero_hits_total]
    G -->|Yes — DB error| I[NOT counted — no increment]
    C & D & E & F & H --> J[GET /metrics — Prometheus scrape]

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Context used:

• Context used - AGENTS.md (source)