Number issues and change requests for human references

Number issues and change requests for human references

A ULID is an unreadable handle, and the 8-character prefix the list showed
is mostly timestamp: this repository's 27 issues shared seven distinct
six-character prefixes and two pairs already collided at eight. The part
people actually type carried almost no information.

Issues and change requests now share one gap-free sequence (#1, #42, ...)
derived from the creation order in the metadata branch's history, the way
one forge counts both in a single run.

The number is not stored. It is the rank of the entity's creation commit,
so nothing is written when an entity is created, a repository needs no
backfill, the same history gives the same numbers on every clone, and
`vole admin index rebuild` reproduces them from git. A ULID stays the
identity: paths, front matter, `closes:` and audit entries are unchanged,
and data/index.db only caches the mapping so a page render does not walk
history.

Every id-taking command accepts a number, `#42` links in a body through a
kind-agnostic /r/<repo>/n/42 route, and --closes resolves an alias to the
ULID before writing.

Closes #45