subscribed-thread routing now respects opt-out decisions

*Junior's message ingress no longer forces users into threads they've declined.*

Thread routing in junior was not checking whether users had opted out of subscribed-thread notifications. @dcramer's #107 adds that decision point to the routing layer, so a user's explicit opt-out now stays opted-out when messages arrive. This fixes the class of bug where routing logic and notification preferences live in separate codebases and sync lazily.

queue diagnostics now surface dropped work

*Silent thread exits in the message queue are now logged.*

@dcramer's #92 instruments the queue-to-thread handoff to detect when work is enqueued but never processed. The diagnostics log which jobs fell through, making it obvious when the thread pool size is too small or a consumer died without error. Before this, dropped messages just disappeared from logs entirely.

MCP tools can now be scoped to plugins

*Plugin-level tool registration prevents tool collisions across integrations.*

When multiple MCP plugins export tools with the same name, junior had no isolation mechanism. @dcramer's #93 adds plugin-scoped tool support, so each plugin can define its own tool namespace without shadowing others. This is table-stakes for a multi-skill agent architecture.

auth guidance now surfaces generically across skills

*Missing authentication no longer requires per-skill error handling.*

@dcramer's #106 refactors auth error responses so any skill can surface the same guidance when credentials are absent or invalid. Previously, each integration had to catch and reformat its own auth failures. Now junior speaks a single dialect of "you need to authenticate."