Phase 2: an assignee is a person you can click

Phase 2: an assignee is a person you can click

`assignees:` was in the frontmatter, written by the tool and read by nothing:
not validated, not set by any command, not in the index, not on a page. It now
goes the whole way — the field is checked at the push boundary, the index holds
it in an `issue_assignees` table mirroring `issue_labels`, and the web renders it
as a filter link on both the list and the detail page.

The field is held to the *roster's* account-name rule rather than the label
rule. It names a person, so the label rule (which allows dots) would accept a
value no account could ever have; a capital or a `@` prefix is a typo that would
render as a link to nobody. It is deliberately NOT resolved against the roster at
push time: a clone may be offline, an assignment may name someone who has not
been created yet, and a name in an issue file is a claim on the same footing as
`author` — the hook's job is the format, not the org chart. Stored as written
rather than as an account id, for the same reason `author` is: the file stays
readable in a plain clone, and because names are never reissued an assignment
keeps meaning the same person after a rename.

The index needed a schema version to go with the new table, because
`EnsureIssues` only re-reads when the ref moves: an existing instance would have
served rows derived from the old shape until someone happened to push. A bump
now empties the derived issue rows and leaves every repo's `issues_sha` empty, so
the next read rebuilds from git. The index is disposable; a stale row is worse
than a rebuild.

`vole issue new -a`, `assign`, `unassign`, `list --assignee` and `%(assignees)`
are the CLI half. An assignment that changes nothing is not a commit — `Update`
reports `ErrNoChange` and the branch does not move — so a script that reruns
stays idempotent without growing a linear log.

Verified: TestAssigneeRuleMatchesAccountNames compares this package's regex with
identity.ValidateName over the edge cases (a name the roster accepts and the
format rejects would only ever show up as a rejected push);
TestAssigneesAreAccountNames drives valid, capitalised, dotted, `@`-prefixed,
too-short, too-long and space-bearing values through the reader;
TestAssigneeRoundTrip proves the writer emits a sorted one-per-line list that the
reader reads back; TestSchemaVersionDropsRowsFromAnOlderShape writes rows an
older build would have left, reopens, and checks they are gone, `issues_sha` is
empty and a second open does not re-wipe; and
TestIssueAssigneesRenderedFilteredAndValidated plus the CLI end-to-end test push
an assignment over SSH, click it, filter by it and get a push rejected for
`Wilbur` with the path and value named and the branch unmoved. scripts/ci.sh
green (gofmt, vet, build, markdown links, go test -race ./...).