feat(tabs): add opt-in redesigned session tabs

[luoxuanzao]

Summary

• Adds an opt-in New session tabs interaction behind a new switch in Settings → Experimental (off by default); with it off everything behaves exactly as before.
• With the switch on, session tabs show the conversation title with a hover close button, new sessions and conversations picked from history open in their own tab instead of replacing the active one (at the tab limit a notice points at the setting that raises it), and the row scrolls — following the active session — instead of squeezing every tab. The new-conversation button that reset the active tab and its New session (in current tab) command stay out of the way; /clear still resets the current session.
• Why: numbered tabs are unreadable and the new-conversation button silently replaced the active session; the switch keeps this a no-risk opt-in.

Verification

• npm run typecheck
• npm run lint
• npm run test (169 suites / 3593 tests)
• npm run build
• npm run release:check
• npm run audit:prod (0 vulnerabilities)
• Tested affected qodercli behavior against a real CLI when applicable — N/A, UI/tab behavior only
• Real device: Obsidian 1.13.7 dev vault over CDP — switch off: numbered tabs, pencil button, history replaces the active tab; switch on (toggled through the settings UI): titles + hover ×, history opens a new tab, limit notice; measured geometry 24×24 (legacy) vs 180/138×30 (pills).

Safety

• No credentials, private vault content, internal URLs, or personal paths are included
• User-visible changes are documented in CHANGELOG.md