Write full documentation with screenshots

README covers what CodeNavigator is, screenshots of each view, quick
start, configuration, keyboard shortcuts and links to docs/: a city
guide (what every visual means, getting around), architecture (indexer,
sessions, GitHub, rendering passes, routing), the HTTP API, GitHub App
setup including the alert permissions, deployment (Docker, systemd,
nginx, limits, security notes) and development (tests, layout,
conventions, design notes).

docs/CITY.md

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+# City guide
+
+Switch to **City** in the toolbar. The city is laid out from the repository's folder tree at human
+scale, so a large repository is a large city.
+
+## Reading the city
+
+| You see | It means |
+|---|---|
+| A **district** (big block, name floating above its tallest tower) | A top-level folder |
+| A **block** inside it, separated by streets | A nested folder; deeper folders have narrower streets |
+| A **building** | A file |
+| Building **footprint** | Lines of code |
+| Building **height** | Complexity (branching) |
+| **Lit windows** | Definitions: more functions and classes, more lights on |
+| Building **colour** | Semantic layer (application, tests, vendor, …), switchable palette |
+| Name on the **roof** and **wall** | The file name |
+| **Green street sign** sticking out at a corner | The folder that block belongs to; each corner of an intersection names its own folder |
+| **Smoke** | The file has open medium/low security alerts |
+| **Fire and smoke** | The file has an open critical/high security alert |
+| **Yellow tape** rising across the walls | Dependabot alert (on the manifest file) |
+| **Red tape** falling across the walls | Code scanning alert |
+| **Light beam** into the sky | A search result |
+| **Cyan chevrons** on the road | Route to a file the selected file imports (**amber** if the import was inferred) |
+| **Pulsing magenta dashes** on the road | Route from a file that imports the selected file |
+| **Arcs** in the sky | The same import relationships, as the crow flies |
+| **Green figures** | Residents. They don't mean anything; they just live there |
+| **Wall with posters** | The city limits, with a few facts about the codebase pasted on |
+
+## Getting around
+
+**Helicopter (`1`)**: drag to orbit, Shift+drag (or right-drag) to pan, scroll to zoom.
+Double-click a building to fly to it, or double-click a street to drop down and walk.
+
+**Walk (`2`)**: click the map to capture the mouse and look around, `WASD` to move, `Shift` to run,
+`Esc` to release the mouse. The crosshair names the building ahead. Press `E` (or click) to open
+it in the sidebar. Walk right up to a building and its source code appears on the wall. You can't
+walk through buildings or out past the city wall.
+
+**Fly (`3`)**: like walking, but `Space` and `C` climb and descend, and moving follows where you
+look.
+
+**Touch**: left thumb is a joystick, right thumb looks around, pinch zooms (or climbs when flying).
+
+**Minimap**: click it to jump there. It shows your heading, the city wall, security alerts and
+the selected file's routes.
+
+## Following dependencies
+
+Select a file (click it, search for it, or pick it from the sidebar). Routes appear on the streets
+to everything it imports and from everything that imports it. With a file selected, switching to
+**Walk** puts you at its door facing along the first route: follow the arrows.
+
+## Tour and links
+
+- **Tour (`T`)**: flies over the six largest districts with captions, and stops as soon as you
+  touch anything.
+- **Copy link**: copies a URL with the repository, view, camera position and selected file.
+  Opening it loads the repository and puts you in the same spot. Links work for GitHub
+  repositories only.
+
+## Settings
+
+The gear opens settings: landscape palette, source text on tiles, **city residents** (they keep
+the city animating, so switch them off on slow machines) and a frame-rate monitor. The sidebar
+folds away with the tab on its left edge, or `[`.