Bring back channel templates on Agents2 teams - #868

Stacked on #867 (teams).

Brings back channel templates on top of Agents2 teams. A template is a name, the teams a new channel starts with, and a starting Canvas.

• Storage: templates are saved on this device in buzz.channel-templates.v1, the same way teams are (features/channel-templates/local.ts). Relay storage is deferred: teams don't fit the kit team record, which has no instructions field and is read per community. We'll revisit both together.
• Settings → Channel templates: create, edit and delete templates (new card in the channels plugin).
• Create channel: a Template select appears when any templates exist. Picking one sends the existing channel setup: the members of its teams who are your agents in this community, plus the Canvas. The session applies, confirms and retries that setup as before. Teams deleted since are skipped. A template that would add nothing here creates a plain channel.

Not brought back: group default templates, Save as template…, picking individual agents.

Validation

• Full vitest run at 5b01707: 579 files, 8239 tests passed. New tests in local.test.ts, ChannelTemplateSettings.test.tsx and CreateChannelDialog.test.tsx.
• tsc and design:check are clean.
• An independent agent review ran. Fixed from it: an empty template no longer sends setup, a template deleted mid-draft no longer stays selected, a blank Canvas is not seeded, and an edit keeps team ids that aren't on this device.
• Not yet exercised in the running app. Deferred to a native test: both themes, narrow/wide widths, keyboard.

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