Issues and requests are frontmatter, not JSON

Issues and requests are frontmatter, not JSON

issues/<ulid>/issue.md and requests/<ulid>/request.md replace meta.json plus
description.md, and approvals/<account>.md replaces approvals/<account>.json.
Every file on refs/heads/vole is now frontmatter markdown read by one package, so
there is one reader and one writer for issues, requests, comments and approvals
instead of a JSON path and a hand-rolled key/value path.

The diff is the interface. A JSON array rewrites its neighbour to add a comma, so
one label cost two changed lines and removing one cost three; a block sequence
costs one. docs/spikes/issue-frontmatter measured the same six edits at 25 changed
lines as meta.json and 10 as frontmatter, and one file per entity means one path
to diff, blame and log.

Three rules make YAML safe to hand-edit rather than something to police:

- the writer always double-quotes free text, so no indicator, reserved word or
  " # " can change what a title means
- a label is quoted only when the parser would not read it back bare ("null"),
  which leaves ordinary labels as they were
- a reader refuses an unquoted title that the parser read differently: "title:
  fix #12 crash" would otherwise be stored as "fix" and nobody would find out

Control characters are refused on both sides, so a tree the server accepts is a
tree it can always rewrite.

`updated` is gone from issues, requests and the index's issues table. It cost a
changed line on every edit — 11 of the 21 frontmatter lines in the spike's
sequence — and the commit that changed the file already carries the time.

Verified: TestRenderParseRoundTrip puts 13 titles that break naive YAML (hash,
colon, leading dash, null, quotes, backslash, unicode, 190 bytes) through the
writer and the reader; the server tests push a rendered issue over SSH and render
it, and the approval-ownership tests withdraw approvals/<account>.md. Then
scripts/ci.sh green (gofmt, vet, build, markdown links, go test -race ./...).