Add support for continuing existing conversations in non-interactive mode

[bachittle]

Currently, each gemini CLI invocation starts fresh without memory of previous interactions.

Proposed: Add --continue (-c) or --resume (-r) flag to maintain conversation context across CLI calls. This would allow users to build on previous responses without re-explaining context.

Example:
gemini -p "Explain X"
gemini --continue -p "What about Y?"

this infrastructure already exists in interactive mode, so this should be pretty easy to transfer over:

• /chat save - Save current conversation
• /chat resume - Resume saved conversation
• /chat list - List saved conversations

but then -p commands would need to save every time it is used. so would need to think about this carefully.

[mbiss10]

+1 would really benefit from this!

[bl-ue]

My PR #4401 has support for just this, among other things:

PS C:\dev\gemini-cli> gemini -p "Repeat what I say backwards: Hi there"
Loaded cached credentials.
ereht iH
PS C:\dev\gemini-cli> gemini --resume -p "Goodbye"
Loaded cached credentials.
eybdooG

You can also specify a session number, e.g. --resume 123.