The daemon writes the agent register under the name it should have, not the one it has #2549

Observed

EmrgServer.__init__ builds the daemon's agent registry from the register's canonical
path, not from the one the register is actually at:

# emrg/server/daemon.py:872
self._agents_log = agents_file(runtime_dir)

That handle is both read and written: _register_agent calls AgentRegistry.for_dir,
which runs on every session open (emrg/server/daemon.py:2931, :3847).

Every other register reader in the package goes through
emrg.config.register_for_reading (#2545), which answers the file that exists -- a
register still carrying its pre-rename name projects.yml is a state
migrate_legacy_registry documents and tolerates: a read-only home, a permission
boundary, a concurrent caller that got there first each leave the legacy file exactly
where it is, and the daemon still starts.

AgentRegistry._read answers a missing file with [] (emrg/server/agents.py),
and for_dir writes back what it read. So on such a host the first session opened reads
"no rows" and creates a new agents.yml holding exactly the one row it just
registered.

register_for_reading prefers the new name whenever it exists (deliberately, so an
older daemon that re-creates the legacy file cannot shadow the current register), so
from that moment every reader in the package answers the one-row register and every
entry left in the legacy file is gone from view -- silently: nothing raised, nothing was
logged, and _read never saw those rows at all.

Why this is the same defect as #2548, one call site over

#2548 removed exactly this shape from the scheduler's self-heal and left the daemon's
handle alone on the argument that the startup migration moves the register before the
server object exists. That argument holds on a healthy host and fails on the host the
derivation exists for: the migration is documented to be able to fail, and
register_for_reading was introduced because that state is reachable.

Expected

The invariant #2548 stated -- read and write address one file, and it is the file the
register is actually at -- must hold for the daemon's registry too.

What the fix must satisfy

1. On a host whose register is at projects.yml and whose migration cannot complete,
   opening a session adds its row to that file and creates no agents.yml.
2. On a host whose register is at agents.yml (every healthy host) behaviour is
   unchanged, and so is the case where neither name exists yet: the row lands in
   agents.yml.
3. A test constructs the first state from a temporary directory, and is red without the
   change.