Architecture Decision Record index and template

[codebestia]

Create docs/adr/README.md plus docs/adr/0000-template.md, and backfill ADRs for the decisions this codebase has already made and re-litigated in review: one canonical devices table, dropping sequenceNumber for (createdAt, id) ordering, ciphertext-only message storage, per-device envelopes over a shared ciphertext, the Phase-1 sealed box → Phase-2 Signal migration path, and MLS for groups.

Acceptance criteria:

• A template with context / decision / consequences / status is provided
• At least the six decisions named above are backfilled, each stating what was rejected and why
• The index lists ADRs with status (accepted / superseded)
• Cross-linked from IMPLEMENTATION_DOCS.md

[Muhd010]

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[teejayboy4tony-sketch]

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[josephchimebuka]

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[Damola09]

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Can i work on this?

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[josephchimebuka]

Addressed in #610 (retargeted to dev after #609 was auto-closed for targeting main).

[github-actions]

Closed by #610, merged into dev.

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