feat: add codex template support

[xmtp-coder-agent]

Resolves #85

Summary

• Adds coderTemplateNameCodex config option (env: CODER_TEMPLATE_NAME_CODEX, default: task-template-codex)
• When creating a task, selects the codex template if the issue title contains "codex" (case insensitive) or has a "codex" label
• Passes issue title and labels through the webhook router → dispatcher → handler pipeline

Changes

• config.ts: New coderTemplateNameCodex field with default task-template-codex
• schemas.ts: Added coderTemplateNameCodex to HandlerConfig
• webhook-router.ts: Extract issueTitle and issueLabels from assigned issue payload
• handler-dispatcher.ts: Pass new config and context fields through
• handlers/create-task.ts: resolveTemplateName() selects codex template when appropriate
• Tests: 4 new test cases covering title match, label match, case insensitivity, and default fallback

Test plan

• All 191 tests pass
• Typecheck passes
• Lint + format pass
• New tests cover: codex in title, codex label, case insensitive match, default template fallback

Note

Add Codex template selection to CreateTaskHandler based on issue title and labels

• Introduces a coderTemplateNameCodex config field (env: CODER_TEMPLATE_NAME_CODEX, default: task-template-codex) alongside the existing coderTemplateName.
• CreateTaskHandler gains a resolveTemplateName() method that picks the Codex template when the issue title contains "codex" (case-insensitive) or any label equals "codex"; otherwise falls back to coderTemplateName.
• WebhookRouter.routeIssuesAssigned now populates issueTitle and issueLabels (normalized from label objects to strings) in the CreateTaskContext so the handler can inspect them.
• HandlerConfig and the dispatcher are updated to propagate coderTemplateNameCodex and the new issue context fields through the handler pipeline.

^{Macroscope summarized c1d1acf.}