Update docs; add contributing standards

- docs/overview.md: rewrite for current architecture (src/ layout,
  split JS/CSS modules, credentials/models/functions/ui config
  categories, correct test fixture targets)
- docs/contributing.md: new — documentation philosophy and style guide
- AGENTS.md: add rule to follow docs/contributing.md

docs/contributing.md

@@ -0,0 +1,57 @@
+# Documentation Standards
+
+## Philosophy
+Prefer self-documenting code and in-code comments over external documentation.
+Write external docs only when the information cannot live closer to the code.
+
+## What belongs where
+
+| Content | Where |
+|---------|-------|
+| Why a function does something non-obvious | Inline comment at the relevant line |
+| Contract, parameters, exceptions | Docstring on the function |
+| Module purpose, dependencies, exports | File header comment |
+| Architecture, data flow, system-level decisions | `docs/overview.md` |
+| Setup, configuration, usage for humans | `README.md` |
+| Agent/AI session rules | `AGENTS.md` |
+
+## Style
+- Brief and technical. No preambles, no summaries, no filler.
+- No emojis.
+- High-level overview only in external docs — implementation details belong in code.
+- Present tense, imperative mood for instructions.
+
+## Python docstrings
+All public functions use Google style: one-line summary, then `Args:`, `Returns:`, `Raises:` sections if non-trivial. List every domain exception the function can raise directly or via propagation.
+
+```python
+def crop_save(src: Path, dst: Path, box: tuple[int, int, int, int]) -> None:
+    """Crop src image to box and write to dst, overwriting if present.
+
+    Args:
+        src: Source image path.
+        dst: Destination path; parent directory must exist.
+        box: (x, y, w, h) in pixels.
+
+    Raises:
+        FileNotFoundError: If src does not exist.
+    """
+```
+
+## JS file headers
+Every JS file starts with a block comment stating: purpose, dependencies (Depends on:), exports (Provides:).
+
+```js
+/*
+ * helpers.js
+ * Pure utility functions with no dependencies on other application modules.
+ *
+ * Provides: esc(), toast(), isDesktop()
+ */
+```
+
+## When NOT to document
+- Do not add docstrings or comments to code you did not change.
+- Do not document implementation details that are already clear from the code.
+- Do not keep comments that describe what the code does when the code already says it clearly.
+- Do not update `docs/overview.md` for implementation details — only for structural/architectural changes.