fix(team): fall through to cached catalog when live slash-command list is empty

[bbt567]

Root cause

In team mode, a backend-native slash command such as /compact is silently treated as ordinary text and wrapped in the ## New Messages wake payload, so the agent never receives the bare command and replies conversationally instead of executing it.

resolve_slash_catalog degrades live → cached → static. The live arm short-circuits on any Ok result from the session, including an empty list:

Ok(items) => {
    let names = items.into_iter().map(|item| item.command).collect();
    return Some((names, SlashCatalogSource::Live));   // returns even when empty
}

A backend's live slash_commands can be transiently empty before its discovery handshake completes. For the claude backend, ClaudeSessionBackend::capabilities() merges discovered_caps.slash_commands only once the control_request{initialize} control response lands (sniff_control_initialize); until then it is []. Returning that empty live list yields CatalogEmpty, which disables every team slash command and forces the wrapped-wake fallback — even though the same commands are already present in the persisted agent_metadata.available_commands snapshot (chain arm b).

Evidence (real captured data, not inference)

Reproduced on v2.1.41+ with a team whose Leader is claude (transport: CliAssumed) and a member is codex.

1. The failure reply is generated by the agent, not AionCore. The string `/compact` 是内置命令… does not exist anywhere in this repo. The persisted claude thinking block shows the agent reasoning about it but not executing:

"The user typed /compact which is a built-in CLI command… I should acknowledge this is a built-in command and not invoke any skill…"

2. The same backend executes /compact correctly when sent bare (non-team). A direct (non-team) claude conversation returns Not enough messages to compact. — i.e. the native compact path ran. The team path differs only in that the command is wrapped, so claude never sees a bare /compact.

3. The persisted snapshot already advertises compact. agent_metadata.available_commands for the claude agent (2d23ff1c) contains {"name":"compact","description":"Free up context by summarizing the conversation so far"}. So arm (b) of the chain would resolve the command — it is never reached because arm (a) returns the empty live list first.

Fix

Adopt the live catalog only when non-empty. An empty live list falls through to cached/static. The decision is extracted into live_catalog_or_none so the degradation boundary has a unit test (the chain previously had none — which is how this slipped in). A backend that genuinely advertises no commands still degrades correctly: the cached snapshot is empty or the chain ends at None, both yielding the wrapped wake.

This makes no claim about any external CLI's behavior — it only stops a transiently-empty live list from masking a populated persisted snapshot.

Verification

• cargo check -p aionui-app ✅
• cargo test -p aionui-app --lib → 18 passed (incl. new live_catalog_adopted_only_when_non_empty) ✅
• cargo test -p aionui-team --test e2e_team_flow → 29 passed, 0 failed — all existing catalog tests (recognized_command_is_sent_bare_to_member/lead, unrecognized_slash_and_plain_text_fall_back_to_wrapped_wake, catalog_unavailable_falls_back_to_wrapped_wake, empty_catalog_logs_warn_and_falls_back_to_wrapped_wake) still green ✅

Closes #3750.