docs(memory): clarify that memory does not auto-compact when full

The "Persistent Memory" callout said "when memory is full, the agent
consolidates or replaces entries to make room," which reads as if the
store self-compacts automatically. It does not: the `memory` tool
returns an overflow error and the agent does the consolidation in-turn
(the design from #41755). Also note that `replace` is bound by the same
limit — swapping in a longer entry can still overflow — which is the
exact case that confused a user (replace rejected near the cap even
though the math was correct).

website/docs/user-guide/features/memory.md

@@ -20,7 +20,13 @@ Two files make up the agent's memory:
 Both are stored in `~/.hermes/memories/` and are injected into the system prompt as a frozen snapshot at session start. The agent manages its own memory via the `memory` tool — it can add, replace, or remove entries.
 
 :::info
-Character limits keep memory focused. When memory is full, the agent consolidates or replaces entries to make room for new information.
+Character limits keep memory focused. Memory does **not** auto-compact: when a
+write would exceed the limit, the `memory` tool returns an error instead of
+silently dropping entries. The agent then makes room itself — consolidating or
+removing entries in the same turn before retrying (see [What Happens When Memory
+is Full](#what-happens-when-memory-is-full)). Note that `replace` is also bound
+by the limit: swapping an entry for a longer one can still overflow, so the new
+content must be shortened (or another entry removed) to fit.
 :::
 
 ## How Memory Appears in the System Prompt