feat(buzz-acp): add auto permission mode

[skyerus]

Closes #2884.

What this solves

buzz-acp can't be put into Claude Code's auto permission mode. PermissionMode has five variants and auto isn't one, so BUZZ_ACP_PERMISSION_MODE=auto fails clap validation at startup and the value never reaches the adapter — even though the adapter advertises it.

Probing claude-agent-acp 0.61.0 over ACP, session/new returns auto first in the mode config option:

   - auto | Auto
   - default | Manual
   - acceptEdits | Accept Edits
   - plan | Plan Mode
   - dontAsk | Don't Ask
   - bypassPermissions | Bypass Permissions

Why auto is the mode this harness wants

buzz-acp auto-approves every session/request_permission with allow_once (acp.rs:1068, acp.rs:1671), which is correct for a chat-driven harness — there's no human at a terminal, and forwarding a prompt would hang the turn.

The side effect is that default and acceptEdits become indistinguishable from bypassPermissions, leaving only plan and dontAsk as modes that constrain anything, and both are too restrictive for real work. auto is the missing middle: a classifier reviews each tool call, with no prompt for anyone to answer.

Implementation

Two lines of behaviour — the enum variant and its wire string:

/// Auto-approve tool calls, with a classifier reviewing each one.
#[value(alias = "auto")]
Auto,

// as_wire_str()
Self::Auto => "auto",

The #[value(alias = "auto")] form matches the existing style (Default and Plan carry the same redundant-but-explicit alias). Doc comments on the enum and on the --permission-mode arg updated to mention it.

Tests

Extended the four existing per-mode tests rather than adding new ones, so auto is covered everywhere the other five are:

• test_permission_mode_wire_strings — "auto"
• test_permission_mode_is_default — not the default
• test_permission_mode_value_enum_kebab_case — "auto" parses
• test_permission_mode_value_enum_camel_case_aliases — alias path

cargo test -p buzz-acp --lib permission_mode
test result: ok. 7 passed; 0 failed

cargo fmt -p buzz-acp -- --check clean. cargo clippy -p buzz-acp --lib --all-features produces no new warnings (the two it reports are pre-existing, both in queue.rs).

Testing it manually

BUZZ_ACP_PERMISSION_MODE=auto buzz-acp --relay-url … --private-key …

Startup log should read permission_mode=auto where it previously rejected the value.

Deferred

Not addressed here: whether the adapter honours auto identically to the interactive CLI. agentclientprotocol/claude-agent-acp#607 reports permissions.defaultMode: "auto" in settings.json having no effect, though that's a different mechanism from session/set_config_option — which is the path this uses and the one the adapter advertises. Worth verifying end-to-end before relying on the classifier as a security control.