* feat(coding-agent): ingest external entries in in-memory sessions
SessionManager.inMemory() takes the entries to start from, so a session
held outside the filesystem can be resumed as it was stored.
Replaying through appendMessage/appendCompaction cannot do this: ids are
minted fresh, so every stored firstKeptEntryId, targetId and fromId has to
be remapped, timestamps are rewritten, and parentId always comes from the
current leaf, so a branched tree needs the original branch() choreography
reproduced call by call.
The entry-adoption half of _setSessionFile becomes _loadEntries, which the
constructor also uses when there are entries but no file. Entries carrying
a session header keep it; without one, the header comes from the options
and the entries become its body.
* fix(coding-agent): carry the stored version through restored entries
inMemory() took SessionEntry[], which excludes SessionHeader, so an
external store could not pass the header it saved and the header branch of
_loadEntries was unreachable through the public API. A restore therefore
always synthesised a current-version header, and the migration that ran
next read that header and skipped: a v2 session kept its hookMessage roles
forever.
Widening to FileEntry[] lets the stored header through, version included.
Migration moves inside the header branch, where a version exists to compare
against; without a header the entries are adopted as current-version, which
is the only thing they can mean.
