feat: auto-create Slack channels when new squads or memory entities are added

[kokevidaurre]

Problem

When squads-cli creates new directories — squads (squads add <name>) or memory entities (e.g., new entries under .agents/memory/customer/<name>/) — there's no automatic creation of a matching Slack channel. Users who run a Slack-integrated workflow have to create channels manually each time, which means new entities go un-discussed and team awareness lags.

Current state

• The Slack bot exists (bin/squads-slack) and can post to existing channels.
• Channel IDs are tracked in private memory (squad-company, squad-cli, etc.).
• No integration point between directory-creation events in the CLI and channel-creation in Slack.

Proposed behaviour

When the CLI creates a new persistent entity (squad, or new memory subdirectory under a configured root), it should optionally trigger Slack channel creation with a predictable naming convention (e.g. squad-<name> for squads, <entity-type>-<name> for memory entities).

Design questions to resolve

Trigger point

• Option A: New --slack flag on relevant commands (squads add <name> --slack)
• Option B: Project-level config in .squads/config.yml — e.g. slack.auto_create_channels: true
• Option C: Post-create hook in .squads/hooks/post-add.sh that users can wire to anything

Auth

• Slack bot token via SLACK_BOT_TOKEN env var
• API: conversations.create with name and is_private — needs channels:manage scope
• Channel ID needs to persist somewhere for future posts (memory file or config file)

Failure handling

• If Slack API call fails (rate limit, name collision, missing scope), the directory creation should still succeed and the failure should be logged — never block local work on Slack availability

Naming

• Slack channel names: lowercase, no spaces, max 80 chars, hyphens/underscores
• Need a transform from the user-supplied name (e.g. Client Alpha → client-alpha)
• Collision strategy: append -2, -3? Or fail with a clear message?

Out of scope

• Two-way sync (Slack channel deleted → local dir deleted): dangerous
• Auto-archiving channels when entities are removed: defer
• Posting an introductory message in the new channel: nice-to-have, not core

References

• Slack API: https://api.slack.com/methods/conversations.create
• Squad scaffolding: src/commands/create.ts

Acceptance criteria

• User can opt in to Slack channel auto-creation via one of the trigger options above
• Channel is created with predictable naming
• Channel ID persists for use by future Slack posts
• CLI command succeeds even if Slack API fails (graceful degradation)
• No entity names exposed by default — opt-in only