feat: add --session/--keep-session/--url-only options

[atani]

Summary

Adds three new options to gh-attach so that it can be used as a drop-in
replacement for bespoke playwright-cli wrappers against GHE (git.pepabo.com)
behind OneLogin SAML:

• --session <name> — reuse a named playwright-cli session (e.g. a
  persistent ghe session that has already been SAML-authenticated). The
  previous behavior was to always use the unnamed default session, which
  meant session-only cookies and "remember this device" state were not
  shared across invocations.
• --keep-session — skip playwright-cli session-stop on exit. Keeping
  the browser process alive preserves session-only cookies, dramatically
  reducing the frequency of SAML re-authentication prompts.
• --url-only — print uploaded asset URLs to stdout (one per line) and
  skip comment creation/update. Lets callers post evidence via their own
  tooling (e.g. gh pr review --comment --body-file) while still using
  gh-attach for the upload step.

Motivation

On GHE behind OneLogin SAML, the existing flow forced a fresh SAML login
many times per day because the browser session was torn down after every
upload. With --session ghe --keep-session, a single morning login keeps
working for the whole day.

Bumps VERSION to 0.7.0.

Test plan

• gh-attach --help lists the three new options
• bash -n bin/gh-attach passes syntax check
• gh-attach --session ghe --keep-session --url-only --issue N --image foo.png --host git.pepabo.com --repo OWNER/REPO uploads without tearing down the session and prints only the URL
• Existing Browser/Release/Direct modes without the new flags behave unchanged

[atani]

GHE は React 実装のため file-attachment カスタム要素が存在せず Direct mode が使えない。Browser mode の URL extraction も GHE の textarea 構造と非互換。結局 dotfiles 側の wrapper を playwright-cli 直接実行方式に戻したので、このPRの変更は GHE 用途には不要。github.com 用途では有用 (future use) なので、別PRで本家に出す形にする。