docs: ADR 0003, card revisions with a list of parents, merged per side

Records step 10's decisions with their reasons and the alternatives weighed: a list of parents instead of ADR 0001's single parentRevisionId (a revision with two parents resolves a conflict), a three-way merge per side against the newest common ancestor (no silent merge inside a side), and an explicit fractional position for the order. ADR 0001 gets an "Amended by" line, and AGENTS.md points to the new ADR.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

docs/adr/0001-offline-sync.md

@@ -9,6 +9,9 @@
   longer holds. The decision and its rules stand. Deleted content is redacted before
   deletion (ADR 0002, rule 2), and how authorship is secured in shared spaces is decided
   with shared spaces (ADR 0002, *Consequences*).
+- **Amended by:** [ADR 0003](0003-card-revisions.md) (2026-09-30): rule 2's revisions have a
+  list of `parents` instead of `parentRevisionId` and merge per side (front, back); cards
+  have a fractional `position`.
 
 ## Context