[feat-006] Park add_citation tool path; restore <CITATIONS> JSON prompt

Empirically the add_citation tool route was unreliable on Olava - model
wrote [N] markers but skipped the tool call, so no citations rendered.
Reverting the system prompt to the original <CITATIONS> JSON block format
restores the proven path. The add_citation tool stays defined and
dispatched (events still emit if called), and collectTurnCitations still
prefers tool-emitted citations when present - so re-engaging this path
later is just a prompt change.

Defensive parser improvements kept: page schema simplified to a string
(was oneOf, which not all tool-call parsers handle cleanly), and
normalizeCitation now coerces string-of-digits markers to integers.
Both make the legacy <CITATIONS> parser more robust to model output
variation as well.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>