docs: single-source normative rules with marked canonical copies (refs #64)

[Coldwings]

Refs #64

What

PR 2 of 4 under #64: gives every normative rule exactly one canonical home and makes the remaining duplicates explicit, machine-checkable copies. Audience separation is preserved — AGENTS.md for agents, CONTRIBUTING.md for community, docs/README.md for readers; only the normative text is single-sourced.

Changes

1. Marker convention (established here, enforced by scripts/check-docs.sh in PR 4):
   • <!-- CANONICAL id="..." --> at a rule's single home;
   • <!-- CANONICAL-COPY source="..." id="..." --> around safety-critical verbatim copies — identical modulo leading indentation, edit both or neither.
2. Canonical homes assigned:
   • the verify trio → AGENTS.md (marked copies in CONTRIBUTING.md and docs/README.md, so each audience still sees the commands in place);
   • the Hash64/score wire-protocol rule → docs/hashring.md §6; the ChunkKey rule → docs/chunk.md §6 (marked copies in AGENTS.md — safety-critical redundancy an agent must not miss, now explicitly labeled).
3. "Authority and canonical sources" section in AGENTS.md: precedence design-assumptions.md > package docs > AGENTS.md > CONTRIBUTING.md, with the newest-accepted-ADR tiebreak from PR docs: establish the Architecture Decision Record system (+3 backfills) (refs #64) #65; CONTRIBUTING.md points at it.

Deliberately out of scope: marking every shared paragraph (the golden-value rule etc.) — the precedence clause covers unmarked duplication; over-marking would ossify prose.

Verification

Docs-only. go vet ./... clean; go test ./... -race -count=1 13/13 PASS.

[Coldwings]

@copilot please review. Focus on: marker-format consistency (ids/sources pairing), whether the copy blocks truly match their canonical sources (modulo indentation), and the precedence clause's clarity. Do NOT push commits; post review comments only.

[Copilot]

@copilot please review. Focus on: marker-format consistency (ids/sources pairing), whether the copy blocks truly match their canonical sources (modulo indentation), and the precedence clause's clarity...

Review finding: the marker IDs/sources pair correctly, and all three copy blocks match their canonical bodies after indentation normalization. The precedence clause is ambiguous for docs/README.md: it says conflicts among “repository documents” follow an ordered list, but README is neither docs/design-assumptions.md, a docs/<pkg>.md package document, AGENTS.md, nor CONTRIBUTING.md. A conflicting normative rule there has no stated rank. Please either include it in the ordering or scope the clause to the enumerated documents.

[Coldwings]

@copilot fixed: docs/README.md now explicitly ranks between design-assumptions.md and the package documents (the policy whose template governs them). Please re-review.

[Copilot]

@copilot fixed: docs/README.md now explicitly ranks between design-assumptions.md and the package documents (the policy whose template governs them). Please re-review.

Re-review complete: the precedence clause now explicitly ranks docs/README.md and resolves the reported ambiguity. No further findings.