chore: use singular plugin directory

[hadrien]

Problem

• The shared FastSQLA plugin source uses an unnecessarily plural parent directory.

Solution

• Rename plugins/fastsqla to plugin/fastsqla.
• Update Claude, Codex, and Context7 source paths.
• Preserve both providers’ required hidden manifest directories.
• Record the singular project convention for future packaging updates.
• Verify native validators and real marketplace installations.

fs-1rc.5

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.36%. Comparing base (931d47c) to head (cd890ec).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #114   +/-   ##
=======================================
  Coverage   97.36%   97.36%           
=======================================
  Files           1        1           
  Lines         114      114           
  Branches        2        2           
=======================================
  Hits          111      111           
  Misses          3        3           

Flag	Coverage Δ
fastapi-post121	97.36% <ø> (ø)
fastapi-pre121	97.36% <ø> (ø)
sqlmodel	97.36% <ø> (ø)

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[hadrien]

Independent review found no runtime or discovery blockers. It identified one low maintenance risk: Codex’s scaffold helper defaults to plural plugins/ and could regenerate the old path. Added the explicit project convention to AGENTS.md in cd890ec; no other findings.