# `muse commit --sign` fails silently when no identity is resolvable for a human commit

## Background

Companion to issue #1. Even once #1's gate bug is fixed, there's a second,
independent inconsistency in the same code path in
`muse/cli/commands/commit.py`:

```python
signing = None
pre_signer_public_key = ""
if sign and resolved_agent_id:
    from muse.cli.config import get_signing_identity
    signing = get_signing_identity(root, agent_id=resolved_agent_id)
    if signing is not None:
        _, pre_signer_public_key = encode_public_key(signing.private_key)
    else:
        logger.warning(
            "No signing identity found for agent %r — commit will be unsigned. "
            "Run `muse auth keygen && muse auth register` to set up a keypair.",
            resolved_agent_id,
        )
```

When `--sign` is passed **with** `--agent-id` but that agent has no
registered key, the `else` branch fires and a clear warning is logged.

When `--sign` is passed **without** `--agent-id`/`MUSE_AGENT_ID` at all
(the human case), the entire `if` block was skipped before #1's fix — so
there was no path to ever reach the warning, regardless of whether a human
identity existed or not. Once #1 changes the gate to `if sign:`, a human
commit with **no** identity configured will correctly reach
`get_signing_identity()`, get `None` back, and hit the same `else` —
except the warning message hardcodes agent phrasing ("No signing identity
found for agent %r"), which reads wrong when `resolved_agent_id` is `""`.

## Fix

Branch the warning message on whether `resolved_agent_id` is set, so a
human commit with no identity gets human-appropriate phrasing instead of
`No signing identity found for agent ''`:

```python
elif resolved_agent_id:
    logger.warning(
        "No signing identity found for agent %r — commit will be unsigned. "
        "Run `muse auth keygen && muse auth register` to set up a keypair.",
        resolved_agent_id,
    )
else:
    logger.warning(
        "No signing identity found — commit will be unsigned. "
        "Run `muse auth keygen && muse auth register` to set up a keypair."
    )
```

## Acceptance criteria

- `muse commit -m "..." --sign` as a human with **no** identity configured
  prints a clear, human-phrased warning (not `agent ''`) and the commit
  succeeds unsigned, same as today's agent behavior.
- `muse commit -m "..." --sign --agent-id X` with no key for `X` still
  prints the existing agent-phrased warning, unchanged.