docs: fold completed FIXDRY audit into a decision log

[AprilNEA]

All checkboxes in FIXDRY.md are done, so the tracking file has served its purpose. The durable part — the rationale for infrastructure we deliberately keep custom instead of replacing with crates — moves to docs/DECISIONS.md as a decision log; the completed checklist history stays available in git.

[greptile-apps]

Greptile Summary

This PR moves the completed dependency audit into longer-lived docs. The main changes are:

• Deleted the completed FIXDRY.md checklist.
• Added docs/DECISIONS.md with the retained custom-infrastructure decisions.
• Updated the pinned rust-overlay entry in devenv.lock.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.
• The removed checklist is not required by repository automation.
• The decision log keeps the durable rationale from the deleted tracking file.

Important Files Changed

Filename	Overview
FIXDRY.md	Removed the completed audit checklist after its durable decisions were moved into the decision log.
docs/DECISIONS.md	Added a decision log entry for infrastructure that remains custom instead of being replaced by crates.
devenv.lock	Updated the pinned oxalica/rust-overlay revision and hash used by the development environment.

_{Reviews (1): Last reviewed commit: "docs: fold completed FIXDRY audit into a..." | Re-trigger Greptile}