feat: reins group / ungroup — create, edit and dissolve tab groups

[karngyan]

Stacked on #34. Review that one first; this PR's diff is against feat/tab-groups.

What

This is the write side of tab groups. The agent can mark the tabs it works in, or organize the user's tabs when asked. reins still never groups anything on its own.

reins group --tab 12 --tab 13 --title reins --color orange   # new group → "group 7"
reins group --tab 14 --group 7                               # add to an existing group
reins group --group 7 --collapse                             # edit: title / color / collapse / expand
reins ungroup --tab 12                                       # take tabs out
reins ungroup --group 7                                      # dissolve the group; tabs stay open

• Protocol:
  • GroupTabsParams, UpdateGroupParams (needs at least one prop), and UngroupTabsParams (exactly one of tabIds/groupId).
  • Colors are the nine Chromium ones.
  • All three methods are tier read, because grouping touches the tab strip, never page content. Readonly sites can be grouped; denied ones can't.
• Extension handlers:
  • A new group is pinned to the first tab's window via createProperties.windowId. Chrome's default is the focused window, which would drag tabs across windows.
  • Props are only sent when set, so collapsing doesn't reset the title.
  • Chrome's own errors (bad group id) pass through unchanged.
• Policy gate:
  • group_tabs/ungroup_tabs check every target tab's host. For ungroup --group, that means the group's current tabs.
  • The first denial refuses the whole call and tags the audit record with that tab.
  • The checked set is pinned into params, and ungroupTabs only accepts that set, so the handler can't act on a tab the gate didn't check.
  • The unsupported check runs before any lookup, because on a browser without groups tabs.query({groupId}) may ignore the filter and return every tab.
  • update_group only edits the label and has no host check.
• CLI:
  • reins group covers both create/add and edit through a small methodFor hook on ToolCommand.
  • Validation: --collapse and --expand together, a missing --tab/--group, and an unknown color are all usage errors.
• Docs:
  • SKILL.md documents the commands and adds one line of guidance: you can group the tabs you open for a task so the user sees which are yours, but don't regroup the user's tabs unless asked. It also notes that grouping moves tabs into the group's window.
  • Web commands page and a minor changeset.

Test plan

• pnpm build && pnpm test && pnpm typecheck && pnpm lint: all green (515 tests).
• Unit tests:
  • Handlers: new group in the first tab's window, adding to an existing group, props only when set, ungroup, empty set is a no-op, unsupported.
  • Gate: every tab checked, one denied tab refuses all and tags its tabId, ungroup --group resolves the group's tabs once and ungroups exactly those, update_group ungated, schema validation, unsupported before any policy check.
  • CLI: build, method selection and every usage error.
• Live in Dia, only on tabs reins opened: create, collapse, rename, recolor, ungroup, dissolve. The groups show in Dia's vertical sidebar. Of the nine colors, only cyan changes: it comes back as blue.
• Live in Chrome, only on tabs reins opened:
  • open two tabs → group (title + color) → groups/tabs show them → collapse → expand + rename + recolor → ungroup one tab → dissolve (tabs stayed open) → close.
  • Error paths: bad group id, a missing flag, an invalid color.
  • Re-run after the review fixes.
• Not live-tested: a denied site, because adding a deny rule from the CLI can only be undone in the popup. Unit tests cover it.

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