fix(schema): accept channels: in campaign schema (#296)

[Tomas2D]

Fixes #296.

Problem

The runtime reads campaign.channels at every DESIGN/FINDINGS gate and POSTs a markdown gate summary to each configured channel:

• orchestrator/iteration.py → from orchestrator.channels import notify_gate; notify_gate(channels, ...)
• orchestrator/channels.py documents the shape (kind ∈ {slack, webhook}, url, webhook_url, headers).

But orchestrator/schemas/campaign.schema.yaml never declared a channels property while setting additionalProperties: false at the top level. So nous run aborts during pre-flight validation:

Campaign validation error: Additional properties are not allowed ('channels' was unexpected)

The documented feature is therefore impossible to use — the campaign is rejected before the run ever starts, and nous schema campaign doesn't list channels.

Fix

Add a channels property to the campaign schema whose item shape mirrors exactly what orchestrator/channels.py reads:

• kind — enum [webhook, slack], optional (the dispatcher loop defaults it to webhook).
• url — target URL for a webhook channel.
• webhook_url — incoming-webhook URL for a slack channel.
• headers — optional extra HTTP headers for a webhook channel.

additionalProperties: false on each entry catches field typos, matching the strictness of the rest of the schema.

Tests

Adds regression tests under TestCampaignSchema in tests/test_schemas.py, so the schema and runtime can't drift apart again:

• test_channels_accepted — a channels:-bearing campaign (slack + webhook) validates.
• test_channels_default_kind_accepted — kind is optional.
• test_channels_invalid_kind_rejected — bad enum value rejected.
• test_channels_unknown_field_rejected — typo'd channel field rejected.

$ pytest tests/test_schemas.py tests/test_nous_schema_command.py
63 passed

(nous schema campaign now lists channels with its description.)