* fix(memory): persist the DeerMem extraction watermark across restarts and replicas
The updater's per-thread extraction watermark lived only in an in-process
LRU, so a restart or a second Gateway replica on the same storage root fed
already-extracted turns to the memory LLM again.
FileMemoryStorage now keeps it in users/{user}/agents/{agent}/.metadata/
watermarks.json, keyed by thread id, written atomically under the per-user
lock without touching memory.json and capped by watermark_max_keys. Each
update feeds after the furthest of the LRU entry and the persisted one, and
writes through after an extraction or a pre-screen skip; bypass_watermark is
unchanged and sidecar IO failures fall back to the LRU. Memory clears remove
the file with the other .metadata sidecars. Content-based message identities
are stored as SHA-256 digests so the sidecar holds no conversation text.
* docs(changelog): link the memory watermark entry to #7204 and #7205
* fix(memory): never replace an unreadable watermark sidecar
set_extraction_watermark rewrites the whole watermarks.json, and the shared
sidecar reader turns an OSError into an empty mapping, so a failed read could
replace every other thread's watermark with the current one (or delete the
file). The watermark sidecar now has its own reader: a missing file reads as
empty, malformed content reads as empty (nothing to keep; the next write
repairs it), and any other OSError propagates before anything is written. The
updater already logs that and keeps the extraction successful.
* fix(memory): keep the persisted watermark when the last message has no identity
_message_identity returns None for a last message with neither an id nor
text. The updater wrote that None through, which deleted the thread's entry
from watermarks.json, losing a watermark a peer may have advanced and forcing
a full feed on every replica. The write-through now skips None: the persisted
entry still marks an extracted prefix and the furthest-of-both read keeps
using it.
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