Add atomic parent issue creation - #3134

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.

Token impact

Measured with tiktoken 0.14.0 using o200k_base. Exact billed tokens remain host- and model-dependent.

The fixed increase is entirely in the input schema. Operational savings come from removing the second tool call and its full-parent response.

# Capture actual tools/list output from binaries built at main and this branch.
go build -o /tmp/github-mcp-server ./cmd/github-mcp-server
go build -o /tmp/mcpcurl ./cmd/mcpcurl
/tmp/mcpcurl --pretty=false \
  --stdio-server-cmd '/tmp/github-mcp-server stdio' schema > /tmp/tools.json

# Verify the relevant behavior and request counts.
go test ./pkg/github \
  -run 'TestIssueWriteCreateWithParent|Test_AddSubIssue|TestGranularCreateIssue' \
  -count=1

# Tokenize compact tools/list and representative call/result JSON.
python3 -m venv /tmp/token-count
/tmp/token-count/bin/pip install 'tiktoken==0.14.0'
/tmp/token-count/bin/python - <<'PY'
import json
import tiktoken

payload = json.dumps(
    json.load(open('/tmp/tools.json'))['result']['tools'],
    separators=(',', ':'),
)
for name in ('o200k_base', 'cl100k_base'):
    print(name, len(tiktoken.get_encoding(name).encode(payload)))
PY

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)