refactor(agents): migrate .agent/ planning tree to .agents/

Standardize on the .agents/ directory (shared with project skills):
phases/, user_stories/, reports/, screenshots/, validate.sh, and
phase-sessions/ + pipeline.log all move to .agents/ (git mv preserves
history; runtime artifacts move alongside).

Updates every reference in AGENTS.md, README.md, .gitignore, app
docstrings, and test story headers. Historical KB content in data/
and the runtime pipeline.log transcript are left untouched.

app/models.py

@@ -1,6 +1,6 @@
 """SQLAlchemy models (PostgreSQL 17 + pgvector).
 
-Data model — see ``.agent/PLAN.md`` §Data Model:
+Data model — see ``.agents/PLAN.md`` §Data Model:
 
 * ``documents`` — one row per imported A9 file (full content, path, sha256 hash).
 * ``chunks``    — retrieval units; each chunk points at its parent document
@@ -65,7 +65,7 @@ from sqlalchemy.orm import Mapped, mapped_column, relationship
 from app.config import get_settings
 from app.db import Base
 
-# Single source of truth for the vector column size (see .agent/PLAN.md A6).
+# Single source of truth for the vector column size (see .agents/PLAN.md A6).
 EMBEDDING_DIM: int = get_settings().embedding_dim