fix(acp): title Codex/ACP sessions with Buzz agent display name

[Bartok9]

Summary

Codex (and similar ACP harness UIs) title sessions from the start of the injected prompt. Every managed Buzz agent shared the same [Base] You are operating inside the Buzz platform… opener, so session lists showed identical truncated base text instead of the real agent name (Fizz, Varys, …).

This wires the managed agent display name from Desktop → buzz-acp and puts a short labeled [Session] section first in:

• session/new systemPrompt framing (protocol v2 / modern path)
• legacy user-message framing (prepend_base_for_legacy + format_prompt sections)

Base-prompt substance is unchanged. Desktop observers that split on [Base] / [System] / [Workspace] keep working — [Session] is an additive leading section.

Closes #2334

Changes

• Desktop (managed_agents/runtime.rs): set BUZZ_ACP_DISPLAY_NAME from display_name, else name
• buzz-acp config: --agent-display-name / BUZZ_ACP_DISPLAY_NAME
• pool: framed_system_prompt + legacy prepend put [Session]\n{name} first; sanitize controls / collapse whitespace / cap 80 chars
• queue: legacy format_prompt injects [Session] before [Base] when present
• Tests: title-before-base, title-before-workspace, sanitize, legacy prepend

Out of scope

• Multi-agent self-ID inside persona body ([Bug] ACP harness does not inject agent identity into system prompt — agents can drift to wrong persona under thread context pressure #2287)
• Notify live ACP procs on rename (Agent rename does not notify running agents: AGENTS.md update invisible mid-session #1823)
• Rewriting base prompt content

Test plan

• cargo test -p buzz-acp --lib session_title
• cargo test -p buzz-acp --lib framed_system_prompt
• cargo test -p buzz-acp --lib prepend_base
• cargo test -p buzz-acp --lib pool::tests (96 ok)

. ./bin/activate-hermit
cargo test -p buzz-acp --lib session_title
cargo test -p buzz-acp --lib pool::tests

AI-assisted; human-reviewed.