docs(archive): preserve PR discussions 3220, 3219, 3218

[AceHack]

Preservation discipline: capturing alignment drift and review friction into native repository memory.

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[Copilot]

Pull request overview

Archival of three merged PR discussions (#3218, #3219, #3220) into the docs/pr-discussions/ preservation surface as native repository memory. These are auto-generated archives produced by tools/pr-preservation/archive-pr.ts capturing PR descriptions and Copilot review comments.

Changes:

• Add PR #3218 discussion archive (minimal-shard tick pattern).
• Add PR #3219 discussion archive (substrate sanity-check tick).
• Add PR #3220 discussion archive (39-candidate manual triage tick).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
docs/pr-discussions/PR-3218-shard-tick-2001z-minimal-shard-pattern-real-dependency-wait.md	Archive of PR #3218 description + Copilot review.
docs/pr-discussions/PR-3219-shard-tick-2015z-substrate-sanity-check-on-main-standing-by.md	Archive of PR #3219 description + Copilot review.
docs/pr-discussions/PR-3220-shard-tick-2018z-39-candidate-manual-triage-via-9-variant-ta.md	Archive of PR #3220 description + Copilot review.

These files live under docs/pr-discussions/**, a history surface where author attribution is preserved as provenance metadata per the closed enumeration in the Otto-279 carve-out. Content is verbatim-archived PR data; no code changes.