[FEATURE]: add session renaming

[GameCat7428]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Add the option to rename sessions manually, like with /rename in the right session or through opencode session rename (new name) (session id)

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• [FEATURE]: Sticky editable session title row at top of TUI (click to rename or /rename) #24579: Sticky editable session title row at top of TUI with click-to-rename or /rename support — very similar to the /rename command request here
• Session rename dialog: auto-select text or start empty for faster renaming #6153: Session rename dialog UX improvements — also about manual session renaming
• [Desktop 1.2.22] Session rename functionality not available in Desktop app #16677: Session rename functionality not available in Desktop app — same underlying feature gap
• [FEATURE]: Allow agents to rename their own session #15346: Allow agents to rename their own session — related session renaming capability

Please review these issues to see if your request is already covered before we proceed.

[lokarex]

It has been supported. You can tpye '/rename' in the session.

[xpufx]

I looked for this a lot on the gui myself. There's a hamburger menu icon next to the circular "context" icon. "rename" is there.

[PVLPM]

You can double-click on the name in the session, and you can rename it after it. C'mon, don't open the issue until you research and ask your LLM first.