[feat] add unbounded session connect and send default

[BegoniaHe]

Preflight

• I searched existing Issues and PRs.
• This matches the current-branch, no-legacy, Python 3.14+ policy.
• I filled exactly one of Human note or Agent note, matching who wrote this Issue.

Problem

/session watch is a time-capped, multi-target subscription (60s–10 days, 16 per actor). Operators who want a durable 1:1 pairing still have to renew watches and type the target UMO on every /send. There is no connect/disconnect that keeps forwarding until explicitly stopped and makes /send use that target by default.

Proposal

Add an in-memory session link owned by the trusted actor, current session as listener:

• /session connect <UMO> creates or replaces a 1:1 unbounded forward from the target into the current session. Requires session.watch on both sessions in the same config.
• /session connect with no argument shows the current link or usage.
• /session disconnect drops that link. Requires session.read, same as /session unwatch.
• After connect, /send [content] (including attachment-only) delivers to the linked target. /send <UMO> [content] still sends to an explicit UMO.
• Authorization is rechecked on every forward and send. Links disappear on process restart. One link per actor+listener; at most 16 links per actor.

Non-goals

• Do not persist links across restarts.
• Do not add Dashboard or OpenAPI management.
• Do not remove or change watch TTL limits.
• Do not add listener targeting for connect (current session only).

Author

Agent

Agent note

Requested in chat after PR #185 review: operators need an unbounded connect that both forwards the target UMO and lets /send omit the UMO. Follow-up PR will be feat/session-connect. Merge still needs a human maintainer review and a separate AI-assisted review.