Add live ACP session config picker

[nori-sessions]

Summary

• Add live ACP session config state and ACP set_config_option plumbing.
• Add /session-config picker for current ACP session select options.
• Extend mock ACP agent and docs for live config options.

Test Plan

• cargo fmt --check
• cargo check -p nori-acp -p nori-tui -p mock-acp-agent --tests
• cargo clippy -p nori-acp -p nori-tui -p mock-acp-agent --tests -- -D warnings
• cargo check --bin nori
• timeout 600s cargo test -p nori-acp test_session_config (timed out locally while compiling/linking test binary)
• cargo test -p nori-tui session_config_picker --lib (timed out locally while linking nori_tui test binary)

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