# Derive the SemVer release tag from pyproject.toml automatically

## Background

`pyproject.toml`'s `version` field must stay in canonical PEP 440 form
(`0.2.0rc15`, no hyphen) because that's the literal string hatchling embeds
in the built sdist's filename — `publish_muse_release.sh` and
`smoke_muse.sh` both derive the tarball URL directly from it. A hyphenated
form (`0.2.0-rc15`) was tried during the rc15 release and broke the publish
script's tarball-filename lookup (hatchling normalizes it away regardless of
what's written in the file), so it was reverted back to `0.2.0rc15`.

Separately, `muse tag add` enforces strict SemVer 2.0, which *requires* the
hyphen before a pre-release identifier (`v0.2.0-rc15` is valid, `v0.2.0rc15`
is rejected with `"Version tag must start with 'v' ... is not valid semver"`).

These two canonical forms are incompatible at the string level — there is no
single literal that satisfies both PEP 440's canonical form and SemVer 2.0's
required hyphen. `pyproject.toml` stays the single source of truth in PEP 440
form (see decision above); the SemVer tag needs to be *derived* from it, not
hand-typed as a second, independently-maintained version string.

## Goal

Whenever a release is tagged with `muse tag add`, the SemVer-hyphenated tag
string is computed from `pyproject.toml`'s PEP 440 version automatically —
no one hand-types `v0.2.0-rc15` and risks a typo or drift from the actual
shipped version.

## Suggested approach

A small helper (Python, using the already-available `packaging` library)
that takes a PEP 440 version string and emits the SemVer-compatible tag:

```python
from packaging.version import Version

def pep440_to_semver_tag(version_str: str) -> str:
    v = Version(version_str)
    pre = f"-{v.pre[0]}{v.pre[1]}" if v.pre else ""
    return f"v{v.major}.{v.minor}.{v.micro}{pre}"

# pep440_to_semver_tag("0.2.0rc15") -> "v0.2.0-rc15"
```

Wire this into the standard release flow (documented in
`musehub/.muse/agent.md`'s "Standard release flow" section) as a step after
`publish_muse_release.sh` succeeds:

```bash
TAG=$(python3 -c "from packaging.version import Version; v=Version('$MUSE_VERSION'); pre=f'-{v.pre[0]}{v.pre[1]}' if v.pre else ''; print(f'v{v.major}.{v.minor}.{v.micro}{pre}')")
muse tag add "$TAG" --message "release $MUSE_VERSION"
```

## Deliverables

- [ ] Helper function (or inline one-liner, whichever fits the existing
      script style better) added to the release flow, not hand-typed.
- [ ] `musehub/.muse/agent.md`'s "Standard release flow" section updated
      with the new tagging step.
- [ ] Manually QA'd once against a real release before considering this done
      (tag the current `0.2.0rc15` release once MWP verb QA — muse#63 — is
      fully signed off, per gabriel's explicit sequencing: manual QA first,
      then tag).

## Out of scope

- Changing `pyproject.toml`'s format — settled as PEP 440, no hyphen, per
  the incident above.
- Retroactively tagging any release prior to this ticket.