fix(rpiv-pi): ellipsis-proof the artifact-path collector

A prose ellipsis is a valid [\w.-]+ string, so an agent's elided reference
to a SIBLING artifact placed after its real announcement won the last-match
scan: run 2026-08-20_18-10-39-17e7's phase 5/9 announced its real path,
then wrote "`.rpiv/artifacts/elaborations/...__phase-4.md`" in prose — the
collector took the elided string, fataled on a file that never existed, and
silently dropped the real 30KB elaboration from the splice.

RPIV_ARTIFACT_PATTERN and rpivBucketCollector's filename segment now use a
tempered class ((?:(?!\.\.)[\w.-])+) refusing ".." anywhere in a segment —
the same hazard FILE_LINE_CITATION_RE guards with its lookbehinds. Replayed
over 2,760 recorded sessions: exactly one divergence (the incident, now
collecting the correct path) and zero regressions — no legitimate slug
carries consecutive dots. The elaborate skill also pins the announcement
contract: the artifact path must be the LAST full .rpiv/artifacts/ path in
the reply; siblings are referenced by basename only.