The register is written at session creation, and a directory is registered as itself #2527

What happens today

emrg/server/daemon.py::_touch_project writes an entry into ~/.emrg/agents.yml, the
register of directories this installation knows, whenever an inbound client message
carries a cwd.

Three normalisations inside it act against "the directory the session runs in is the
entry":

1. os.path.realpath(cwd) first. A symlinked path is collapsed before the entry is
   written, so an entry's path is not the string the session holds. Agent.base_dir
   stores its directory verbatim, so the register and the session disagree for every
   symlinked directory.
2. A subdirectory is merged into its longest known parent. Opening a session in
   repo/src/ when repo is already registered writes no entry for repo/src — it
   only refreshes repo's last_active. The exception is a directory that is itself a
   git root.
3. The home directory is skipped (cwd == os.path.expanduser("~") → return).

Two further consequences:

• The hook fires only on inbound messages, so every session the server creates itself
  never reaches it: the sub-session slot, the boss session, the upgrade session.
  Conversely a client that sends a message carrying a cwd registers it whether or not a
  session was opened there.
• The name is basename and nothing makes it unique. Two different directories both
  named src produce two entries both named src. scheduler._resolve_agent_path(name)
  resolves a task's config.agent by that name, so the second directory silently takes
  over the first one's tasks.

What it must do instead

• A directory a host opens a session in gets its own entry, path verbatim equal to
  str(session.cwd) — no realpath, no expanduser, no trailing-slash
  normalisation.
• Registration happens when the session is created, not when a message arrives.
• Two exceptions, both mechanical: the home directory (a session there would put its
  session directory under ~/.emrg/sessions/, beside the runtime configuration), and any
  directory under an existing agent's .emrg/ — a derived directory, not one the
  host opened.
• Names are globally unique: a second directory with the same basename is name-2, a
  third name-3; the suffix is allocated once, when the entry is created, and is never
  recycled when another entry is removed.
• The registration logic moves into its own module — emrg/server/agents.py::AgentRegistry
  — with no import of the daemon and no server state, so it is testable on its own.
• ensure_local_exclude stays: registration is where a directory learns to ignore
  .emrg/.
• The non-regular-file guard stays: a FIFO, socket or device as agents.yml is neither
  read nor rewritten.

Acceptance

1. A session created in a directory writes one entry whose path equals
   str(session.cwd) exactly — for a symlinked path as well.
2. A second session in the same directory adds no entry and moves last_active.
3. A session created in a subdirectory of a registered directory adds an entry — the
   direct evidence that the parent merge is gone.
4. The sub-session slot, the boss session and the upgrade session add no entry.
5. Two directories with the same basename are named name and name-2, and
   _resolve_agent_path returns each one's own path.
6. The home directory adds no entry.
7. On a non-regular agents.yml, the register is neither read nor rewritten.
8. The existing suite passes.

Out of scope, as its own change: the projects_list → agents_list protocol rename,
which needs the TUI and GUI to ship with the daemon.