Give work and education entries a hierarchy instead of one flat rhythm

[leinss]

The work section read as a wall of text, and the cause was a single rule:
.entry-body > * + * gave every block an identical 8px gap. Four paragraphs of
prose with four bold words between them, all the same weight and spacing, so
nothing separated anything and the eye had no place to rest.

• Section labels inside an entry are written as **label** alone on a line,
  which markdown renders as a paragraph containing only a bold run. Those are
  now styled as signposts: small, uppercase, letter-spaced, muted, with a lot
  of space above and almost none below, so each binds to the paragraph it
  introduces rather than floating between two.
• The label no longer inherits the general strong rule, which had made it the
  highest-contrast thing in the entry and pulled the eye to the signpost rather
  than the content.
• Body copy gets leading-relaxed and a 68ch measure. At the 840px container
  these paragraphs ran to roughly 95 characters, well past what the eye tracks
  comfortably, which was the other half of the problem.
• Date, company and role are set tight as one header unit with a deliberate gap
  before the body. Previously all four shared one rhythm, so an entry had no
  header.
• Entries are separated by a rule, not just space. With space alone the
  boundary between two jobs looked identical to the boundary between two
  paragraphs of one job, so eight years read as a single column.
• Education gets the same treatment, since it shares entry-body.

Also drops the emoji used as icons: next to the page headings on /work,
/projects and /education, in the "See all X" links, and at the start of the
four homepage section headings. An emoji there renders at a different size,
weight and baseline than the text beside it, and it was inconsistent with the
headings that had none. The "See all X" links get a trailing arrow instead,
which inherits colour and weight. The hero's wave stays: it sits inline in a
sentence and reads as voice, not as an icon.

Test plan:

• pnpm run lint and pnpm run build pass, 82 pages
• internal link check passes, none broken
• /work and the homepage rendered in a browser in dark mode and read at
  three scroll positions, not judged from the markup