Say in every prompt that a completion summary is a PR description

[MJohnson459]

docs/DESIGN.md §8 already settles that a done summary "is written to read as
a PR description" — voro pr opens the pull request with that text as the body —
but the agent-facing prompts only gestured at it ("a PR-ready --summary"), and
the voro-cli skill did not mention --summary at all. Agents consequently
returned one-liners like "Implemented X, tests pass", which is the whole PR body
the operator then reviews against. This is a wording change: no schema, state
machine, or scoring moves, and nothing about when a summary is recorded moves.

Every place an agent is told what a summary is now says the shape, in a phrase or
a sentence:

• crates/voro/src/dispatch.rs — the closing paragraph of
  RETURN_PATH_PREAMBLE_TEMPLATE now says the summary is the pull request's
  description and to write it as one ("a short account of what changed and why
  followed by how you verified it, not a status line").
  REWORK_SUMMARY_SENTENCE says the same and keeps its point-by-point demand
  ("a PR body that answers that feedback point by point, one item per point"). It
  stays a single shared constant, so the redispatch preamble's {rework} block
  and rework_message cannot drift. No new brace-wrapped text was introduced, so
  the single-pass renderer is unaffected.
• crates/voro/src/cli.rs — the done usage block and the set --summary
  line in HELP (which is what voro done --help and voro set --help print)
  describe --summary as the PR description; the done warning appends "the
  summary is the PR description pr opens the pull request from" only when the
  summary is the missing half, and the show [incomplete report] nudge says
  the same.
• plugins/voro/skills/voro-cli/SKILL.md — --summary added to the done
  line in the transitions block, plus a sentence on the shape it takes. Edited
  the plugin copy, per CLAUDE.md.
• docs/agent-integration.md — the copy-paste preamble's example, which was
  modelling the bad shape (--summary "Implemented X, tests pass"), now carries
  placeholder text that reads as a PR description, and the done bullet matches
  the wording landed in the prompts.

Summaries stay optional — the new wording describes the shape of one that is
written, never that one must be. docs/DESIGN.md needed no edit: §8 already
licenses all of this and nothing landed contradicts it.

Verification

• New dispatch.rs unit test preamble_says_the_summary_is_the_pr_description
  asserts the plain rendered preamble against render_preamble; the existing
  preamble_tells_a_redispatched_rework_to_answer_the_feedback and
  the_rework_message_carries_the_feedback_and_the_instruction were extended so
  both spellings — render_preamble(…, rework = true) and rework_message(…) —
  assert the PR-description wording alongside the surviving "point by point".
  Prose assertions match on whitespace-normalised text so a rewrap of the
  hand-wrapped constants does not break them.
• New cli.rs test help_describes_a_summary_as_the_pr_description;
  done_warns_only_about_the_missing_half now also asserts the shape note
  appears for the missing summary and not for a missing branch, and
  a_partial_report_is_flagged_incomplete_in_list_and_show asserts the nudge
  names the PR description.
• cargo test --workspace (470 + 1 + 389 passing, 0 failed),
  cargo clippy --workspace --all-targets -- -D warnings, and cargo fmt --all
  are clean.