docs: document i18n ID patterns in core/

Add agents/i18n-patterns.md covering rust-i18n setup, translation file
format, t!() macro usage, key naming conventions, and locale selection.
Remove completed TODO item and add reference in CLAUDE.md.

CLAUDE.md

@@ -129,6 +129,7 @@ The `agents/` directory contains detailed documentation for AI assistants:
 - `rpc-toolkit.md` - JSON-RPC patterns and handler configuration
 - `core-rust-patterns.md` - Common utilities and patterns for Rust code in `/core` (guard pattern, mount guards, etc.)
 - `s9pk-structure.md` - S9PK package format structure
+- `i18n-patterns.md` - Internationalization key conventions and usage in `/core`
 
 ### Session Startup
 

agents/TODO.md

@@ -6,8 +6,4 @@ Pending tasks for AI agents. Remove items when completed.
 
 - [ ] Architecture - Web (`/web`) - @MattDHill
 
-## Docs to Create
-
-- [ ] Document i18n ID patterns in `core/` (localization keys, conventions, where they're defined)
-
 

agents/i18n-patterns.md

@@ -0,0 +1,100 @@
+# i18n Patterns in `core/`
+
+## Library & Setup
+
+**Crate:** [`rust-i18n`](https://crates.io/crates/rust-i18n) v3.1.5 (`core/Cargo.toml`)
+
+**Initialization** (`core/src/lib.rs:3`):
+```rust
+rust_i18n::i18n!("locales", fallback = ["en_US"]);
+```
+This macro scans `core/locales/` at compile time and embeds all translations as constants.
+
+**Prelude re-export** (`core/src/prelude.rs:4`):
+```rust
+pub use rust_i18n::t;
+```
+Most modules import `t!` via the prelude.
+
+## Translation File
+
+**Location:** `core/locales/i18n.yaml`
+**Format:** YAML v2 (~755 keys)
+
+**Supported languages:** `en_US`, `de_DE`, `es_ES`, `fr_FR`, `pl_PL`
+
+**Entry structure:**
+```yaml
+namespace.sub.key-name:
+  en_US: "English text with %{param}"
+  de_DE: "German text with %{param}"
+  # ...
+```
+
+## Using `t!()`
+
+```rust
+// Simple key
+t!("error.unknown")
+
+// With parameter interpolation (%{name} in YAML)
+t!("bins.deprecated.renamed", old = old_name, new = new_name)
+```
+
+## Key Naming Conventions
+
+Keys use **dot-separated hierarchical namespaces** with **kebab-case** for multi-word segments:
+
+```
+<module>.<submodule>.<descriptive-name>
+```
+
+Examples:
+- `error.incorrect-password` — error kind label
+- `bins.start-init.updating-firmware` — startup phase message
+- `backup.bulk.complete-title` — backup notification title
+- `help.arg.acme-contact` — CLI help text for an argument
+- `context.diagnostic.starting-diagnostic-ui` — diagnostic context status
+
+### Top-Level Namespaces
+
+| Namespace | Purpose |
+|-----------|---------|
+| `error.*` | `ErrorKind` display strings (see `src/error.rs`) |
+| `bins.*` | CLI binary messages (deprecated, start-init, startd, etc.) |
+| `init.*` | Initialization phase labels |
+| `setup.*` | First-run setup messages |
+| `context.*` | Context startup messages (diagnostic, setup, CLI) |
+| `service.*` | Service lifecycle messages |
+| `backup.*` | Backup/restore operation messages |
+| `registry.*` | Package registry messages |
+| `net.*` | Network-related messages |
+| `middleware.*` | Request middleware messages (auth, etc.) |
+| `disk.*` | Disk operation messages |
+| `lxc.*` | Container management messages |
+| `system.*` | System monitoring/metrics messages |
+| `notifications.*` | User-facing notification messages |
+| `update.*` | OS update messages |
+| `util.*` | Utility messages (TUI, RPC) |
+| `ssh.*` | SSH operation messages |
+| `shutdown.*` | Shutdown-related messages |
+| `logs.*` | Log-related messages |
+| `auth.*` | Authentication messages |
+| `help.*` | CLI help text (`help.arg.<arg-name>`) |
+| `about.*` | CLI command descriptions |
+
+## Locale Selection
+
+`core/src/bins/mod.rs:15-36` — `set_locale_from_env()`:
+
+1. Reads `LANG` environment variable
+2. Strips `.UTF-8` suffix
+3. Exact-matches against available locales, falls back to language-prefix match (e.g. `en_GB` matches `en_US`)
+
+## Adding New Keys
+
+1. Add the key to `core/locales/i18n.yaml` with all 5 language translations
+2. Use the `t!("your.key.name")` macro in Rust code
+3. Follow existing namespace conventions — match the module path where the key is used
+4. Use kebab-case for multi-word segments
+5. Translations are validated at compile time