feat: native session links and context menu "Open in new tab"

[alexkreidler]

Summary

Improve session navigation so browser-native behaviors work from the session list (sidebar / main list), and expose Open in new tab from the session row context menu.

Motivation

• Modifier-click (e.g. Cmd+click on macOS, Ctrl+click / middle-click elsewhere) to open a session in a new tab only works reliably when the row is a real link (<a href="...">) or equivalent with correct href.
• Today the context menu appears limited to Rename and Delete; users who prefer menus over shortcuts should get Open in new tab there too.

Requested behavior

1. Session rows use proper URLs — Each session entry should resolve to the same canonical session URL the app already uses for deep linking (e.g. path-based /sessions/{id} with any required sticky/query params), implemented as a semantic link where appropriate so Cmd+click / Ctrl+click / middle-click opens in a new tab without custom JS unless necessary.

2. Context menu — Add Open in new tab (wording can match platform conventions) alongside existing Rename and Delete, performing the same navigation as opening the session URL in a new browser tab.

Notes

• Related prior work: URL-based session linking (#180, #225); those enabled shareable URLs and middle-click for some subagent links—this issue is about primary session list UX and context menu parity.

Acceptance criteria

• From the session list, modifier-click opens the session in a new tab with the correct URL.
• Context menu includes Open in new tab and it behaves consistently with modifier-click.
• Normal click still selects/opens the session in the current tab as today.

[alexkreidler]

Also would be great to make all items in the search modal links to so I can open in new tabs