fix: [ENG-2858] correct tool-mode dream candidate-action mechanism in SKILL.md

Section 7's candidate-kind descriptions instructed the calling agent to
"call `brv curate` UPDATE" and to add `<bv-related>` edges. Neither term
matches anything in the codebase:

- `UPDATE` (and `ADD`) appear only in section 4 as part of a list of
  curate-log operation types — they are LOG classifications, not
  syntactic forms. There is no `brv curate UPDATE` subcommand and no
  `--update` flag. A real agent following the previous wording would
  guess at a non-existent syntax.

- `<bv-related>` is not a real element. The bv-topic vocabulary stores
  cross-links in the `related=` attribute on `<bv-topic>` itself
  (`src/server/infra/render/elements/bv-topic/schema.ts:29`), as a
  comma-separated list of `@domain/topic` refs. The fixture
  `test/fixtures/render/sample-topic.html` and the corresponding
  validator test demonstrate this. An agent following the previous
  wording would author HTML for a phantom element and the curate
  validator would reject it.

This commit rewrites the four kind descriptions to name the actual
mechanism: extend the `related=` attribute (with the documented ref
format) and write via the `brv curate` path-exists / `--overwrite`
flow already documented in section 3. Synthesize's new-topic case
correctly notes the path-exists branch does not apply.

Caught by an SKILL.md-only fidelity test — a fresh agent that read
only sections 3 and 7 successfully ran scan → finalize → undo but
could not author the link/merge writes from the section 7 wording
alone.