An ACP session registered the client's cwd for the *tools*
(`_register_task_cwd` -> `register_task_env_overrides`) but never pinned it
for the *prompt*. `agent/prompt_builder.py` reports
`Current working directory: {resolve_agent_cwd()}`, and `resolve_agent_cwd()`
reads the `_SESSION_CWD` contextvar, which ACP left unset — so it fell back
to `TERMINAL_CWD` / the launch dir.
The system prompt therefore advertised one root (commonly
`~/.hermes/workspace`, or the install tree) while the tools were rooted at the
editor's project. When the model emitted a *relative* path the tools resolved
it correctly; when it emitted an *absolute* path built from the advertised
root, the write landed outside the client's workspace and the turn still
reported success.
Observed in Zed/Buzz-shaped usage: the first prompt in a fresh workspace
creates the file under `~/.hermes/workspace/` and answers "Done." The client's
directory is untouched. Later sessions on the same cwd appear to work once a
session cwd record exists, which makes it look intermittent.
`_run_agent` already calls `set_session_vars(session_key=session_id)` inside
its `contextvars.copy_context()`, and that helper's `cwd` argument exists to
"pin the logical working directory for this context". It simply was not
passed. `gateway/session_context.py` and `tui_gateway/server.py` both already
pass it; ACP was the only surface that did not.
Adds a regression test asserting that the resolved cwd *during the turn* is
the cwd the client passed to `session/new`. It fails without this change
(resolving the install tree instead of the client's project).
cca2a2fc8d