fix(octo): ignore bot's own echoes and non-conversation channels inbound

[lml2468]

Problem (found while completing a real Octo binding)

The inbound hub dispatched two kinds of traffic that aren't user messages, each producing a bogus needs_binding outcome and a failed binding-prompt send (the not_friend / missing sender uid errors in the logs):

1. The bot's own messages echo back on its own socket (from_uid == robot_id). Every agent reply looped back in as a new "unbound user" message; the replier then tried to DM the binding link to the bot itself → err.server.bot_api.not_friend.
2. System/command channels. Octo emits e.g. channel_type 8 systemcmdonline on connect (empty from_uid); these slipped past the DM/group-mention gate and got an unsolicited "please bind" reply → missing sender uid.

Both are confirmed against the reference channel implementation (cc-channel-octo processMessage), which filters from_uid === robotId and gates on a supported channel type — guards we were missing.

Fix

In hub.onMessage, before any dedup / dispatch / audit work:

• drop messages where from_uid == inst.RobotID (self-echo / reply loop);
• drop any channel type that isn't a real conversation (DM / group / topic) via new isConversationChannel.

Both are dropped silently — they're not user traffic, so no audit row, no dedup churn, no spurious binding prompt. Real user DMs (from_uid = the human uid, type DM) are unaffected and still dispatch.

Verification

• End-to-end: a bound owner's real DMs already create a chat session and receive agent replies (the octo_chat_session_binding + assistant messages exist) — unaffected by this change.
• TestHub_IgnoresSelfAndSystemMessages: self (from_uid==robot) and a type-8 system message are dropped (no Handle, no replier), while a real DM is dispatched.
• TestIsConversationChannel: DM/group/topic supported; type 8 / 0 rejected.

go vet + go test -race on the octo package clean.

Related observation (not in this PR): the reference also drops streamOn partial messages; our inbound doesn't process partials problematically today, but it's a candidate follow-up.