feat: add GitHub Copilot SDK backend - #3136

Summary

• add copilot-sdk as an opt-in Graphify LLM backend using the official GitHub Copilot SDK
• reuse the existing Copilot login while disabling tools, MCP, skills, memory, hooks, Git operations, telemetry, remote sessions, and persistent session storage
• support semantic extraction, labels, deduplication, PR analysis, and inline image attachments
• keep Python 3.10 support for Graphify core; the optional Copilot extra is gated to Python 3.11+
• replace the conflicted implementation in feat: add GitHub Copilot SDK backend with CLI fallback #2392 with a clean branch based directly on current v8

Verification

• focused Copilot suite: 200 passed, 1 expected opt-in skip
• Python 3.10 compatibility suite: 31 passed, 2 expected skips
• full suite: 5,120 passed and 13 skipped in the restricted run; all 8 DNS/Unix-socket tests blocked by that sandbox passed when rerun with the required access
• authenticated live Copilot extraction: passed against a synthetic one-line fixture
• official SDK runtime download/start/stop: passed on macOS; CI now runs the same smoke on Ubuntu, macOS, and Windows
• generated skill artifacts: 134 matched
• Ruff: passed
• focused Pyright: 0 errors
• uv lock --check: passed
• source distribution and wheel build: passed
• isolated graphifyy[copilot] wheel install, CLI load, and SDK import: passed

Safety notes

• each request uses a fresh isolated client, session, and temporary workspace
• permission requests are rejected and no host file paths are sent for image attachments
• post-dispatch failures are not replayed because the remote outcome can be unknown
• cleanup and runtime setup have bounded behavior, and source is not dispatched after the request deadline

Supersedes #2392.