Add project docs, dev guide, ADR, and doc-writing skills (#5)

## Summary
- Add `doc/developer-guide.md` — full architecture reference (component
diagrams, `EventDescriptor` debounce/heartbeat state machine, timing
behavior, lifecycle flow, build/test details)
- Add `doc/adr/0001-cpp20-linting-testing.md` — first Architecture
Decision Record
- Add `skills/project-docs.md` — documentation placement conventions,
including a new required **link hygiene** section: agents must verify
links to code and to other docs resolve before finishing a doc change,
and dead links are prohibited
- Add `skills/modern-cpp.md`

## Test plan
- [x] Docs only — no code/build changes
- [ ] Review rendering of Mermaid diagrams and links on GitHub once
merged

Author: alexcvc

doc/adr/0001-cpp20-linting-testing.md

@@ -0,0 +1,58 @@
+# 1. C++20 baseline, linting/formatting tooling, and test strategy
+
+## Status
+
+Accepted
+
+## Context
+
+TickGuard is a header-only C++ library intended for embedded-adjacent targets
+(no dynamic threading frameworks, minimal heap use). It needs a consistent,
+low-friction toolchain so that contributors and CI reviewers judge code
+against the same bar, without requiring bespoke build-system integration for
+every tool.
+
+## Decision
+
+- **Language standard**: target C++20 exclusively (see `CMakeLists.txt`,
+  `CXX_STANDARD 20`). Newer standards are not adopted until the project
+  explicitly revisits this decision.
+- **Formatting**: `.clang-format` is Google-based but deliberately diverges
+  from stock Google style — Allman braces, 4-space indent, 120-column limit,
+  `ReflowComments: false`. Do not "fix" the codebase back to stock Google
+  style; the divergences are intentional.
+- **Static analysis**: `.clang-tidy` enables the broad `*` ruleset with a
+  short, explicitly-documented list of disabled checks and rationale for each
+  (see the file's own header comments — e.g. `-llvm-header-guard` because the
+  project uses `#pragma once` everywhere by design, not `#ifndef` guards).
+  A small `WarningsAsErrors` allowlist (`modernize-use-nullptr`,
+  `cppcoreguidelines-no-malloc`, etc.) is treated as a hard gate. Naming
+  conventions (camelBack methods/variables, `m_`-prefixed CamelCase members)
+  are enforced via `readability-identifier-naming` `CheckOptions` — see
+  `AGENTS.md` for the human-readable version of the same rules.
+- **Linting is external to the build, by design**: `CMakeLists.txt` only sets
+  `CMAKE_EXPORT_COMPILE_COMMANDS ON` to produce a `compile_commands.json`.
+  There is intentionally no `add_custom_target` wrapping clang-tidy or
+  clang-format into `cmake --build`. Lint/format checks run manually or in CI
+  against the generated compile database, keeping the local build fast and
+  tool-version-independent.
+- **Testing**: Catch2 v2.13.10, fetched automatically via `FetchContent` in
+  `tests/CMakeLists.txt` (no manual install step), run through `ctest`. Tests
+  inject time manually (`ev.tick(Xms)`) instead of sleeping wherever possible,
+  reserving real `sleep_for`-based tests for the small surface that actually
+  exercises the live worker thread (`EventSupervisorTests.cpp`).
+
+## Consequences
+
+- Contributors run clang-tidy/clang-format against `compile_commands.json`
+  before submitting; neither is enforced automatically by `cmake --build`.
+- Enabling a new clang-tidy check or disabling an existing one requires
+  updating the rationale comments at the top of `.clang-tidy` — undocumented
+  suppressions should be treated as review blockers.
+- No parallel or competing lint/format configuration should be introduced;
+  `.clang-tidy` and `.clang-format` are the single source of truth, and
+  `AGENTS.md` restates their naming rules for quick human/agent reference
+  rather than redefining them.
+- Bumping the language standard past C++20, or replacing Catch2, is a new
+  decision and should get its own ADR rather than a silent change to
+  `CMakeLists.txt`.