feat(opencode): show the model an opencode session runs in its tile and split headers

An opencode tile showed only the session name, while Claude Code, codex and
DeepSeek tiles add `· <model>`: opencode declared no `modelDetect`, so its
screen was never read for a model and it is launched without a model param.

opencode draws the model on its composer's agent row, directly above the
box's bottom edge: `┃  Build  Big Pickle OpenCode Zen`. Read from its own
1.3.0 source, the row is the agent, the model's name, the provider's name and
`· <variant>` when the model has one, and only colour tells model from
provider. So the field is all of it, exactly what opencode itself shows (the
owner's choice over a short id that only appears after the first reply).

- The pattern anchors on that row sitting directly above the `╹` edge, ends
  the field at a double space (where the 200-column layout's sidebar shares
  the row), skips the `No provider selected` placeholder, and takes the LAST
  such row in the window through a lookahead, so a composer-shaped row the
  agent prints higher up can never stand in for it. A test with a forged pair
  inside the window fails without the lookahead.
- It reads 8 rows: the home screen puts up to five rows of opencode's own
  chrome under the composer (key hints, a tip, the cwd/version row). The
  schema's `screenLines` bound goes from 4 to 8, the reader's own cap; the
  comment there records why a taller window is only safe with such a pattern.
- A permission prompt or shell mode hides the row; the last model is kept.

Measured against every captured opencode 1.3.0 frame (home screen and in
session, 40/60/120/200 columns, mid-turn and at rest, permission prompt):
the model was read everywhere it is drawn and nowhere else. Live on an
isolated instance from this branch, a restored opencode session published
`displayModel: Big Pickle OpenCode Zen` (source: screen) and its tile header
rendered `oc-home · Big Pickle OpenCode Zen`. The owner's own home-screen pane
on the 1.36.0 beta reads the same.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/wiki/Tile-Grid.md

@@ -58,7 +58,7 @@ Each tile has a small header: `● [logo] name · model ......... ⋯ ⤢ ×`
 | `●`    | The session's state: working, idle, waiting on you, needs you (red, and the tile's border pulses), error, ended. Hover the header for how long. |
 | logo   | Which agent runs in the tile (Claude Code, Codex, DeepSeek, Shell, ...). Hover it for the agent and the model by name. |
 | name   | Double-click to rename the session.                                                              |
-| model  | The model the session runs, when Codeman knows it: what the agent itself reports (it follows a `/model` switch), else the model its own config pins (DeepSeek's route, shown "from config"), else the model it was started with. Nothing when unknown. |
+| model  | The model the session runs, when Codeman knows it: what the agent itself reports (it follows a `/model` switch), else the model its own config pins (DeepSeek's route, shown "from config"), else the model it was started with. Nothing when unknown. OpenCode shows the model and its provider together (`Big Pickle OpenCode Zen`), exactly as its own composer does. |
 | `⋯`    | The session menu: options, open in a new window, close the session.                              |
 | `⤢`    | Zoom: the tile fills the grid; press it again (or `Alt+Shift+Enter`) to get the grid back.        |
 | `×`    | Remove the tile. The session keeps running; close it from `⋯` if you want it gone.                |