Add atomic parent issue creation

[zwick]

Summary

Adds optional same- or cross-repository parent attachment to issue creation through GitHub's existing atomic GraphQL createIssue capability.

Why

The MCP tools previously required separate create and parent-attachment calls, leaving a partial-failure window even though CreateIssueInput.parentIssueId already supports atomic creation.

What changed

• Add parent_issue_number with optional paired parent_owner and parent_repo inputs; omit both parent repository coordinates to use the new issue's repository.
• Resolve user-facing identifiers before sending the parent, labels, and other supported fields in one createIssue mutation, with no attachment fallback.
• Keep the existing REST path for unparented creation; parented creation rejects issue_fields rather than broadening this focused change.

MCP impact

• No tool or API changes
• Tool schema or behavior changed — parented creation now uses one GraphQL mutation; unparented creation retains its existing REST behavior.
• New tool added

Prompts tested (tool changes only)

• "Create an issue under parent issue Ensure we have an identifiable user agent #12 and apply the bug label"
• "Create an issue in octo-org/app under parent issue add support for the push_files tool #34 in octo-org/roadmap"
• "Create an issue without a parent"

Security / limits

• No security or limits impact
• Auth / permissions considered — GitHub validates access to both explicitly named repositories, and GraphQL permission or compatibility errors surface without a non-atomic fallback.
• 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

Lint & tests

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

Docs

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