Add multi-account support with automatic account routing

[drtootsie]

Implements multi-account support allowing users to configure multiple GitHub accounts with automatic switching based on repository context.

Key features:

• Account configuration via JSON file with --accounts-config flag
• Automatic account selection based on organization or repository patterns
• Environment variable expansion for tokens (e.g., ${GITHUB_WORK_TOKEN})
• Three matcher types: org (organization), repo_pattern (wildcards), all
• Backward compatible: single GITHUB_PERSONAL_ACCESS_TOKEN still works
• Default account fallback when no match found

Implementation:

• pkg/accounts: Core account routing logic with Config, Router, and matcher system
• Comprehensive test coverage with multiple matching scenarios
• CLI flag: --accounts-config
• Example configuration in accounts.example.json

Use cases:

• Work + personal GitHub accounts (EMU-friendly)
• Multiple organization access
• Repository-specific authentication

Fixes #1940

🤖 Generated with Claude Code

Summary

Why

Fixes #

What changed

MCP impact

• No tool or API changes
• Tool schema or behavior changed
• New tool added

Prompts tested (tool changes only)

Security / limits

• No security or limits impact
• Auth / permissions considered
• Data exposure, filtering, or token/size limits considered

Tool renaming

• I am renaming tools as part of this PR (e.g. a part of a consolidation effort)
  • I have added the new tool aliases in deprecated_tool_aliases.go
• I am not renaming tools as part of this PR

Note: if you're renaming tools, you must add the tool aliases. For more information on how to do so, please refer to the official docs.

Lint & tests

• Linted locally with ./script/lint
• Tested locally with ./script/test

Docs

• Not needed
• Updated (README / docs / examples)