Agent-chat ships its first non-prompt verb — `/inbox <url>` saves to the operator's Corpus Inbox via Claude Sonnet 4.6 with active-client context; companion architecture spec defines the three-layer cleanup path

The chat surface (`apps/chat/`) has been mechanically functional for weeks — Anthropic SDK in `services/prompt-runner/src/chat-turn.ts`, four-cache-eligible system slabs assembled in `services/workspace/src/chat.ts`, three-mode tool dispatch (chat_answer / chat_propose / chat_invoke), Svelte transcript with accept-proposal cards — but it knew about three verbs (`prompt.draft`, `prompt.improve`, `prompt.apply`) and one context dimension (`record_set_id`). For the funder content corpus workflow the operator is actually in, that meant the chat had nothing to offer. This commit ships the first non-prompt verb: `/inbox <url>` recognized by the model, dispatched as `corpus.inbox.add`, writes to `clients/reach-edu/corpus/inbox/` with the extended frontmatter contract from the Corpus-Inbox-Capture-and-Triage spec. Claude is the default and was the default; what changed is the chat now has something useful to do with the API key it already had. Companion spec `Chat-Context-Awareness-Architecture.md` lands alongside, naming the three v0.0.1 design holes the inbox verb exposed (hand-written verb roster, hardcoded active_client_id in the context slab, ActiveView covers only half the microfrontends) and defining a three-layer migration plan (workspace as context broker → slab-assembly contract → verb registry) that each layer ships independently. Path B per the user's pick — ship the verb tonight, spec the architecture once it's working.