Add scaffolding for a proper tech documentation from everything I could remember

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+# Contributing to CCCMS
+
+## Stack
+Rails 8.1, Ruby 3.2, PostgreSQL 16, ImageMagick 7 (`magick`, not `convert`).
+No Sprockets pipeline for public or admin static assets except
+`admin_bundle.js` — everything else is served straight from `public/` and
+cache-busted via file mtime (`ApplicationHelper#mtime_busted_path`), not
+fingerprinted content hashing.
+
+## Content model
+- **Node** — the tree structure (`awesome_nested_set`-derived, migrating
+  toward a plain `parent_id` model — both may be present during that
+  transition). Holds three separate Page references: `head` (published),
+  `draft` (being worked on), `autosave` (in-progress typing).
+- **Page** — one revision's content. `belongs_to :node`, except `autosave`
+  layers specifically: their `node_id` is deliberately `nil`, which is what
+  keeps them out of `node.pages` (the real revision history) entirely —
+  an autosave is not a revision.
+- **Page::Translation** — Globalize's own generated class (`translates
+  :title, :abstract, :body` on `Page`). Not a top-level `PageTranslation`
+  constant, and it has `page_id`, not a `page` association reader.
+
+## Publishing lifecycle
+There is exactly one path from an edit to a published change:
+`autosave!` (writes into the autosave layer, creating it via
+`clone_attributes_from` on the previous head/draft if it doesn't exist yet)
+→ `save_draft!` (clones the *entire* autosave into the draft layer,
+wholesale — every locale, not just the one just edited) → `publish_draft!`
+(promotes draft to head). Nothing writes to `draft` or `head` directly.
+Locking (`lock_for_editing!`, raising `LockedByAnotherUser`) gates every
+step; a lock can exist with no draft or autosave at all (acquisition is
+deliberately separate from content creation).
+
+## Locale architecture
+Presentation locale (which language the admin chrome or public page
+renders in) and content-target locale (which translation a given screen is
+reading or writing) are two different concerns and never share a
+mechanism. The route's `:locale` segment governs the former only. The
+primary node editor always targets the default locale, enforced via
+`Globalize.with_locale` (never `I18n.locale`, which would leak into
+generated URLs through `default_url_options`). Non-default-locale editing
+uses a separately-named route param (`:translation_locale`), deliberately
+distinct from `:locale` so the two can never be conflated by a link helper.
+Globalize's configured fallback chain is correct for public rendering and
+wrong for editing — anywhere a screen needs to know a translation's real,
+unborrowed state, read the `Page::Translation` row directly rather than the
+locale-dependent attribute accessor.
+
+## Assets
+`FileAttachment` (`app/models/concerns/file_attachment.rb`) is a from-
+scratch Paperclip replacement. `STYLES` is a hash of style name to a full
+ImageMagick argument array (not just a geometry string — some styles need
+`-gravity`/`-extent` alongside `-resize`, which a single string can't
+express). `generate_variants` loops over `STYLES` generically; adding a
+style needs no other code change, only `bundle exec rake
+assets:regenerate_variants` to backfill existing assets. The URL contract
+(`/system/uploads/:id/:style/:filename`) is stable and safe to parse
+directly (the numeric id is always the second path segment) rather than
+reconstructed through model methods.
+
+## Aggregator / shortcode system
+`[aggregate ...]` in page body text is parsed by
+`ContentHelper#aggregate?` into an options hash, then executed by
+`Page.aggregate`. Only `tags`, `children` (`"direct"`/`"all"`, combined
+with the current node), `order_by`/`order_direction`, and `limit` are
+actually consumed.  Only the first `[aggregate ...]` in a given body is
+ever processed (`aggregate?` does a single, non-global match).
+
+## TinyMCE integration
+`extended_valid_elements` must define a parent element and its commonly-
+nested children symmetrically and explicitly together — leaving one
+element on the schema's default rule while the other is explicitly
+redefined can cause content loss on save, even when the child would be
+perfectly valid under the untouched default alone. `valid_classes` denies
+all classes by default (`'*': ''`); anything meant to render needs an
+explicit per-element allowance. `content_style` injects CSS into the
+editing iframe directly — the app's real public stylesheet is never loaded
+there, so anything that needs to render correctly while editing needs its
+own rule supplied this way. Constrained-input features (fixed placement
+choices, for instance) are built as custom toolbar buttons with their own
+dialog and `editor.insertContent()`, not the native image dialog or
+`file_picker_callback` — those bring their own resize/edit affordances that
+would undermine a genuinely fixed set of choices.
+
+## Testing
+Shared test-only convenience methods (that need to operate across many
+model/controller test files) live directly on `ActiveSupport::TestCase`,
+in `test/test_helper.rb` — the framework's own designated extension point
+for exactly this, not a monkey-patch of a production model class.