fix: do not pin a repo in the Hermes Agent MCP entry

The Hermes entry was modelled on OpenCode and appended `--repo <root>` to
args. That analogy is wrong. An IDE window is scoped to one project, so a
baked-in repo matches what the user is looking at. Hermes Agent is a
long-lived assistant that moves between projects, and its stdio schema has
no `cwd`, so the pinned repo is whichever one happened to run `install`.

The result was a silent wrong answer rather than an error. Asked about the
codebase in one project, the agent called `get_minimal_context_tool` without
`repo_root` — the tool docs say it is auto-detected — and `--repo` resolved
the call against a completely different repo. Stats, risk score, and review
priorities all came back for the wrong project, with nothing to indicate it.

Every one of the 30 tools takes an explicit `repo_root`, so dropping the
default costs no capability. Without it the same call fails with "repo_root
does not look like a project root", which the agent can see and correct by
passing the path. A loud failure beats a confident wrong answer.

Verified against a real Hermes Agent install: from an unrelated project, the
same question that previously reported another repo's graph now surfaces the
error and the agent asks which repository to use.