feat: a herdr plugin for the sandbox controls (contrib)

[TakiTake]

Closes #61 — the last of the batch. Stacked on #69 (→ #68 → #67 → #66 → #65 → #63 → #62); only the last commit belongs to this PR.

What

contrib/herdr-plugin/ — link it with herdr plugin link contrib/herdr-plugin:

entrypoint	kind	does
status	popup pane	the pane's container, image, project, pall8t version, herdr pane + sandbox mode, and how many other sandboxes are running
shell	overlay pane	pall8t exec into the pane's container — a second shell beside the agent, which keeps running
rebuild / rebuild-no-cache	popup pane	pall8t build in that sandbox's project directory
stop	action	pall8t stop for the pane's container

Output-producing entrypoints are panes, not actions: an action's stdout lands in the plugin log, a pane lands in front of the person who asked. stop has nothing to show, so it stays an action.

Design notes

• Finding the pane's sandbox is the whole trick, and it's why Ship a herdr plugin for pall8t sandbox actions #61 waited on Label pall8t containers and carry the labels through pall8t ls --json #55: the container carries pall8t.herdr.pane, so the plugin looks it up in pall8t ls --json instead of parsing container names. With no pane in the environment it falls back to a single running sandbox, and refuses to choose among several — attaching someone to another agent's container is worse than an error.
• Thin over the CLI: no socket calls, no reading pall8t's state directory. One contract: ls --json.
• In contrib/, not its own repo, so it versions with the CLI it drives — a change to ls --json and the plugin that reads it can land in one commit.

Verification

Live, against a real labelled sandbox (HERDR_PANE_ID=%42):

container : pall8t-p8t-plugin-live-cdac8d38-28299
image     : pall8t-p8t-plugin-live-cdac8d38:501-20-557ad54a8e87
project   : /Users/…/p8t-plugin-live
herdr     : pane %42, sandbox full

shell   → inside: dev@pall8t-p8t-plugin-live-cdac8d38-28299   (agent still running)
rebuild → built pall8t-p8t-plugin-live-cdac8d38:501-20-557ad54a8e87
stop    → stopped pall8t-p8t-plugin-live-cdac8d38-28299

Error paths too: no sandbox running, and a pane whose container isn't labelled.

Not verified: invocation by a live herdr server — there's no herdr session on this machine, so the manifest's entrypoint shapes ([[panes]] placements, action contexts) are written against herdr 0.8's documented plugin contract rather than observed. Everything the script itself does is exercised above.

🤖 Generated with Claude Code

https://claude.ai/code/session_01YTFLvjR7dWvxFQfSyBY4Zm

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