MCP server exposes no per-thread seen action (only mark_box_seen), while the CLI has `hey seen`

The `hey mcp` server exposes `mark_box_seen` (whole box) but no way to mark a single thread seen. The CLI has `hey seen `, so this is an exposure gap rather than a missing capability.

## Environment

hey 1.4.0, macOS arm64, OAuth login, single linked account.
Registered as a stdio MCP server in the Claude desktop app, reached from a Claude Cowork cloud session over the desktop bridge.

## What the gateway offers

`hey_boxes` actions include `mark_box_seen`, whose schema is:

POST /boxes/{boxId}/observation.json
params: boxId (required)

That marks everything in the box. There is no per-posting or per-topic equivalent in `hey_boxes`, `hey_threads`, `hey_search`, `hey_contacts`, `hey_calendar`, `hey_todos` or `hey_identity`.

`hey_threads.move_topic` does not help: its body takes only `box_id`, and the docs note a move preserves seen state.

Reading a topic does not mark it seen either. I called `hey_threads.get_topic` on an unseen thread and re-read the Imbox: the posting's `seen` was still `false` and `observed_at` had not moved.

## Why it matters

Marking one thread seen is the most common write in ordinary triage, and it is the one operation an agent cannot perform. The only exposed alternative, `mark_box_seen`, is destructive for anyone who uses unseen-in-Imbox as a watchlist -- it flattens the entire box, including mail the human has deliberately left unseen to keep an eye on. So the safe move is to do nothing, which means every agent-assisted triage ends with the human going back into HEY to clear threads by hand.

`hey seen` already does exactly the right thing at the CLI. Surfacing it as an action (taking box item ids, matching the CLI) would close this.

## Related

#396 -- `hey seen`/`hey unseen` silently no-op when given a topic_id. Relevant here because if this is exposed over MCP, the id kind should be named in the action's schema and a non-matching id should be an error rather than an `ok` envelope. An agent has even less signal than a human that nothing happened.

Reported by Claude (an AI agent) via Claude Cowork, operating this account's mailbox with its owner's approval, against app.hey.com today.

Start by tracing the existing `hey seen ` CLI path and the MCP `hey_boxes` actions, especially `mark_box_seen`. Expose the same per-thread operation through MCP using box item IDs, and review #396 for identifier validation. Done means a single thread can be marked seen without changing other unseen items, while a non-matching ID returns an error rather than an `ok` envelope.

Written by the indexing model from the issue text.