shard(tick): 0921Z — sed-bug class discovered; apology + 2 corrective PRs (#3395 fix + #3397)

[AceHack]

Summary

• PR shard(tick): 0903Z — PR #3391 merged; PR #3392 partial-stale + PR #3393 sed portability fix #3394 merged
• Discovered sed-bug class: body col 4 (PR TBD) mentions break line-1-anchored sed
• Apology + clarification on misclassified-as-stale Copilot thread on PR shard(tick): 0911Z — PR #3392 merged; 3rd stale-Copilot reply+resolve #3395
• Corrective PR shard(tick): 0911Z — PR #3392 merged; 3rd stale-Copilot reply+resolve #3395 commit bb41604 (in-flight) + PR fix(shard): 0916Z pipe-row col-5 (PR TBD) + body discipline-name ref restore #3397 (0916Z on main)
• New discipline: @-delimited pipe-anchored sed

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[Copilot]

Pull request overview

Adds a new hygiene-history tick shard (0921Z) documenting the discovery of a sed substitution bug class affecting (PR TBD) replacements, along with an apology/clarification and references to corrective PRs (#3395 and #3397). This continues the project’s tick-driven operational log under docs/hygiene-history/.

Changes:

• Introduces tick 0921Z.md capturing the sed bug class root cause and the updated “@-delimited pipe-anchored” substitution discipline.
• Records status updates for related PRs (#3394, #3395, #3396, #3397) and the updated stale/legit classification outcome.
• Re-arms the cron sentinel entry for continuity.