refactor: standardize channel/conversation ID naming across runtime

[sentry-junior]

Context

PR #546 established a clean naming model in the tool/plugin context layer. This tracks remaining debt not required for that fix.

Canonical model (post-PR #546)

• channelId — raw Slack channel/conversation container (C/D/G), stable state-binding key
• conversationId — opaque Junior session identity; for Slack turns is slack:{channelId}:{threadTs} (the Slack conversation ID); for dispatched runs may be agent-dispatch:{id}
• assistantContextChannelId — optional Slack assistant-panel source channel, output routing hint only, not a state key
• threadTs — Slack thread timestamp, kept separately in plugin API

Convention: Id suffix, not ID.

Remaining debt

• Internal threadId variable names (Slack-shaped slack:D:ts) — align with conversationId where generic
• toolChannelId in ReplyRequestContext — rename to assistantContextChannelId
• sourceChannelId in factory.ts/slack-runtime.ts — rename to assistantContextChannelId
• Add naming spec doc to specs/
• Consider helpers: buildSlackConversationId, parseSlackConversationId

Out of scope

• Renaming persisted assistantContextChannelId storage key (needs migration)
• Renaming correlation.channelId / correlation.threadId (stable, telemetry)