feat: open the board, a change, and its artifacts from the terminal

[danielhanold]

↩ Change 0542 — Open the board, a change, and its artifacts from the terminal

• Plan: 2026-10-08-open-the-board-a-change-and-its-artifacts-from-the-terminal.md
• Results: 2026-10-08-open-the-board-a-change-and-its-artifacts-from-the-terminal-results.md

Adds docket open <board|change|spec|plan|results|pr> [id]. With no id, the change is worked out from the checked-out branch.

• In a shared repository, artifacts open as GitHub pages by default. A new layered config key, open.artifacts: github | local, opens the local metadata file in the OS default app instead.
• Private repositories always open locally. pr always opens the PR URL.
• A local open syncs the metadata checkout first. If the sync is refused, it still opens the file, with a stale note.
• --print prints the target without launching anything. --json returns an OpenResult.

Review (deep tier, chosen because the diff is over 1500 lines)

#	Severity	Finding	State
1	important	The opener could hang on Linux when a launched app inherits the stderr pipe (no WaitDelay)	fixed (dadf8fe)
2	important	No guard asserted that the opener's stdout stays off docket's stdout	fixed (06c3906)
3	minor	Multi-line opener stderr made a multi-line error	fixed (d9328ee)
4	minor	No test checked the JSON keys of a fully populated OpenResult	fixed (d9328ee)