fix(agent): raise combined system prompt cap to 1 MiB

[joahg]

Problem

buzz-agent's session/new handler rejects any combined system prompt (harness prompt + persona + discovered AGENTS.md/skill hints) larger than MAX_SYSTEM_PROMPT_BYTES (512 KiB). The hints builder in crates/buzz-agent/src/hints.rs inlines the full body of every SKILL.md it discovers under ~/.agents/skills (plus .goose/skills, .claude/skills, and AGENTS.md layers).

On a machine with a large personal skill library this overflows the cap. A user with ~76 skills (~517 KiB of SKILL.md bodies) hits:

session/new: combined system prompt exceeds 512KB limit (533647 bytes)

and the agent (e.g. Fizz on the Block staging relay) can't start a session at all.

Change

Raise MAX_SYSTEM_PROMPT_BYTES from 512 KiB to 1 MiB, matching the sibling MAX_PROMPT_BYTES constant in the same file.

This cap guards the buzz-acp ↔ buzz-agent stdio (ACP) boundary; it is not the relay frame size (BUZZ_MAX_FRAME_BYTES / DEFAULT_MAX_FRAME_BYTES), so wire limits are unaffected.

Tests

Updated session_new_rejects_oversized_system_prompt in crates/buzz-agent/tests/fake_llm.rs to use a 1100 KiB payload and assert the new 1024KB limit message. cargo test -p buzz-agent passes (21 unit + integration tests).

Notes

• This raises the guardrail; it does not address the underlying behavior that the full skill bodies are inlined into every session/new. A follow-up could summarize/elide skill bodies or make the cap env-tunable (like BUZZ_AGENT_MAX_TOOL_RESULT_TEXT_BYTES), but that's out of scope here.