chore: Add agentic coding tooling

.crosslink/rules/elixir.md

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+# Elixir Core Rules
+
+## Critical Mistakes to Avoid
+- **No early returns**: Last expression in a block is always returned
+- **No list indexing with brackets**: Use `Enum.at(list, i)`, not `list[i]`
+- **No struct access syntax**: Use `struct.field`, not `struct[:field]` (structs don't implement Access)
+- **Rebinding in blocks doesn't work**: `socket = if cond, do: assign(socket, :k, v)` - bind the result, not inside
+- **`%{}` matches ANY map**: Use `map_size(map) == 0` guard for empty maps
+- **No `String.to_atom/1` on user input**: Memory leak risk
+- **No nested modules in same file**: Causes cyclic dependencies
+
+## Pattern Matching & Functions
+- Match on function heads over `if`/`case` in bodies
+- Use guards: `when is_binary(name) and byte_size(name) > 0`
+- Use `with` for chaining `{:ok, _}` / `{:error, _}` operations
+- Predicates end with `?` (not `is_`): `valid?/1` not `is_valid/1`
+- Reserve `is_thing` names for guard macros
+
+## Data Structures
+- Prepend to lists: `[new | list]` not `list ++ [new]`
+- Structs for known shapes, maps for dynamic data, keyword lists for options
+- Use `Enum` over recursion; use `Stream` for large collections
+
+## OTP
+- `GenServer.call/3` for sync (prefer for back-pressure), `cast/2` for fire-and-forget
+- DynamicSupervisor/Registry require names: `{DynamicSupervisor, name: MyApp.MySup}`
+- `Task.async_stream(coll, fn, timeout: :infinity)` for concurrent enumeration
+
+## Testing & Debugging
+- `mix test path/to/test.exs:123` - run specific test
+- `mix test --failed` - rerun failures
+- `dbg/1` for debugging output
+
+## Documentation Lookup
+```bash
+mix usage_rules.docs Enum.zip/1              # Function docs
+mix usage_rules.search_docs "query" -p pkg   # Search package docs
+```
+