Move archived design docs from .agents/project/ to .agents/projects/ (#5073)

## Summary
- 23 archived design docs landed in the singular `.agents/project/` by
mistake in #4545; the canonical path is `.agents/projects/` (plural).
This PR `git mv`'s them into place (history preserved).
- Updates the three AGENTS.md references that pointed at the old path:
`lib/iris/AGENTS.md`, `lib/zephyr/AGENTS.md`, `lib/fray/AGENTS.md`.

## Test plan
- [x] `git status` shows only renames + 3 one-line doc edits
- [x] `rg '\.agents/project\b'` returns no hits in the tree
- [x] pre-commit hook green

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Co-authored-by: Romain Yon <1596570+yonromai@users.noreply.github.com>

Author: yonromai