Improve default chat and prompt window headers

[molostovvs]

Hi, this proposal suggests a small but useful default header upgrade for agentic.nvim.

Proposal

Update the default agentic.nvim chat and prompt window headers to show compact, stateful session information instead of generic labels like Agentic Chat and Prompt.

The default headers should include the active ACP provider, current agent mode when available, current model with thought/effort level when available, and current context usage percentage when available.

Suggested Default Format

[ACP provider][Mode][Model-Thought level][Context]

Examples:

[Codex ACP][Plan][gpt-5.5-high][56%]
[OpenCode ACP][Build][gpt-5.5][42%]