feat: core types, API layer and stdin parser

[shouze]

Summary

Core data model, GitHub REST API wrappers, and stdin input parsing.

Tasks

• src/types.ts — SearchResult, ParsedResults, EpicMetadata, EpicConfig, ChecklistItem, DiffResult, DispatchGroup, GitHub API types
• src/api/api-utils.ts — fetchWithRetry with rate-limit / 429 / 503 handling, paginatedFetch
• src/api/github-api.ts — createIssue, updateIssue, getIssue, addComment, listLabels, listIssueTemplates, listOrgTeams, getTeamsForRepo, getCodeowners, searchIssues, createSubIssueLink
• src/input/stdin.ts — readStdin, detectFormat, parseMarkdown, parseJson, parseResults

Input format

Markdown:

- [ ] `owner/repo` — `path:line` — text
# Replay:
github-code-search "TODO" --org acme

JSON: flat SearchResult[] or { items, replayCommand } wrapper.

Acceptance criteria

• Unit tests in src/api/api-utils.test.ts and src/input/stdin.test.ts pass
• Both markdown and JSON formats parse correctly