refactor: remove legacy mkOpencodeRules alias and opencode-rules compat entry

- Remove mkOpencodeRules backward-compat alias from lib/coding-rules.nix
- Remove opencode-rules alias from lib/default.nix
- Update shells/opencode.nix to use mkCodingRules / coding-rules
- Remove backward-compat test from tests/lib/coding-rules-test.nix
- Update AGENTS.md and modules/home-manager/AGENTS.md docs
- Apply nix fmt formatting to shared-options.nix

modules/home-manager/AGENTS.md

@@ -163,7 +163,7 @@ The agent system was migrated from embedded `agents.json` to file-based canonica
 | `coding.opencode.externalSkills` | `coding.agents.opencode.externalSkills` |
 | Agents embedded in `config.json` | File-based `~/.config/opencode/agents/*.md` |
 | Model hardcoded in `agents.json` | Per-machine `modelOverrides` |
-| `mkOpencodeRules` | `mkCodingRules` (old name still works) |
+| `mkOpencodeRules` | `mkCodingRules` |
 
 ### Migration steps