docs: align repository guidance with the code it describes

[Azganoth]

Summary

AGENTS.md, CONTRIBUTING.md, and docs/ had drifted from the code they
describe, and two conventions the code follows uniformly were written down
nowhere.

Documentation that no longer matched the code

• patterns.md taught the hand-written SETTINGS_PERSISTED_KEYS array as the
  persisted-state pattern. chore: derive persisted state sanitizers from value contracts #157 replaced it with contracts derived from one
  shape and updated decisions.md, leaving the implementation guidance pointing
  at a pattern with no remaining instance in src/, and at the defect the
  decision record exists to prevent: satisfies PersistedTauriStoreKey<State>[]
  checked that each listed key was valid, never that every field was listed.
• architecture.md listed four leaf features; oxlint.config.ts enforces five.
  diagnostics was missing from the paragraph that asks for the two to be kept
  in step, and plugins/ from the subdirectory list.
• src/AGENTS.md advertised types/ as a standard feature subdirectory, which
  no feature has and which the next rule restricts to an exception, while
  omitting services/ (five features) and plugins/ (fifteen modules).
• patterns.md named three of the five directories under src/test/, and
  called factories/ fixtures while a separate fixtures/ directory holds the
  literal sample data.

Guidance moved to where it is read

• Rust import grouping moves from the contribution guide's command reference to
  src-tauri/AGENTS.md. Nothing enforces it, since the rustfmt options are
  nightly-only, so it is applied by hand on every Rust edit.
• src-tauri/AGENTS.md gains a route into patterns.md, whose Tauri Boundaries
  section carries backend-side rules and had none. src/AGENTS.md gains routes
  into Path Identity and Error Handling, the two remaining patterns with no
  entry point; path comparison shipped a defect in Canonicalized Windows paths break folder-context identity comparisons #146.
• The documentation no longer routes readers to the AGENTS.md files, which
  agents load directly.

Conventions nothing stated or enforced

• Command payload structs carry #[serde(rename_all = "camelCase")] and
  boundary errors carry #[serde(tag = "kind", ...)], across 23 and 11
  declarations with no exception. Clippy does not inspect serde attributes, so
  an omission compiles, passes the backend checks, and ships snake_case keys the
  frontend never reads.
• All 50 reactive store reads select one field and all 223 non-reactive reads go
  through getState, but a no-selector call type-checks, passes the lint set,
  and subscribes the component to every field.

Standing rules land alongside: not weakening the verification oracle to obtain
passing checks, backend filesystem side effects, and interaction and
accessibility, the last paired with a new Keyboard Traversal section so the rule
and its reasoning arrive together. The constants and spawn_blocking rules are
reworded to match what the code does.

Related Issue

Not applicable.

Verification

pnpm exec oxfmt --check passes on all six files. Documentation-only with no
executable configuration touched, so no application suite, per the
documentation-only rule in CONTRIBUTING.md and AGENTS.md.

• All six patterns.md anchors referenced from the two AGENTS.md files
  resolve to real headings, as do both new decisions.md links.
• The leaf feature list matches oxlint.config.ts exactly.
• No *_PERSISTED_KEYS constant remains in src/. The surviving
  PersistedTauriStoreKey<State>[] mention is deliberate, describing the removed
  pattern to explain the defect.

No changelog entry. Nothing here changes user-facing behavior.

Notes

Shared Foundations is a new section holding the path and error-handling
pointers. It and Resource Lifecycles are arguably one category under two
headings, and merging them is a reasonable review outcome.

Deliberate omissions:

• The nested deep-merge constraint stays a consequence in decisions.md rather
  than becoming a pattern. No nested setting exists, and documenting it in
  patterns.md would repeat the types/ mistake fixed here.
• File naming, backend logging, and mutex poisoning stay undocumented. Naming is
  uniform across ~150 files and recoverable from any sibling; the other two have
  nine call sites and one.

oxlint.config.ts still restricts @/components/dialogs, a directory removed in
449cf405. architecture.md was corrected for this in e3bb86ab and the config
was not, so it carries a dead boundary root, the mirror image of the
diagnostics drift fixed here and a breach of the same invariant. Left out as it
is not a documentation change.

src/AGENTS.md line 20, the Radix exception in Focus Visibility, and the Radix
mention in Keyboard Traversal are three coupled lines that go stale together if
the Base UI spike in #167 lands.