feat(slack): add /cc slash command gateway and harden command parsing

[lethanhson9901]

Summary

This PR improves the Slack integration in three related areas:

1. Add native Slack slash command support through a /cc gateway
2. Harden @mention command parsing for non-standard whitespace
3. Update Slack setup and usage documentation

What Changed

Native Slack slash command gateway

Slack integration now handles Socket Mode slash_commands events and routes /cc ... into
the existing cc-connect command engine.

Examples:

• /cc help
• /cc model
• /cc status
• /cc mycustom

This keeps Slack-native command handling aligned with the existing built-in, custom command,
and skill command paths.

More robust mention command parsing

@cc_connect /model now parses correctly even when Slack inserts non-ASCII whitespace, such
as:

• non-breaking spaces
• newlines after the mention

This avoids falling through to normal chat handling when the user intended to run a command.

Alias compatibility for slash-style input

Alias resolution now also works when the input is slash-prefixed, so Slack gateway input
like /cc 帮助 can still resolve to alias-backed commands.

Documentation updates

docs/slack.md now documents:

• required commands OAuth scope
• how to create a /cc slash command in Slack
• the three supported Slack input styles:
  • native slash command: /cc ...
  • mention flow: @cc_connect /model
  • DM plain-text fallback: /model

Why

Before this change:

• Slack users could not use a native slash command path for cc-connect commands
• direct Slack /model usage was confusing because Slack interprets it as a native Slack
  command
• mention-based commands were brittle when Slack emitted unexpected whitespace
• docs did not clearly explain the Slack-specific command model

This PR makes Slack command usage more reliable and much easier to understand.

Implementation Notes

• Handles socketmode.EventTypeSlashCommand in the Slack platform
• Uses /cc as a gateway command and normalizes the payload into the existing engine
  command format
• Reuses the same session key model as other Slack message flows
• Keeps replies as normal channel messages rather than ephemeral responses
• Limits native slash-command handling to /cc to avoid overreaching Slack app
  configuration assumptions

Tests

Added/updated tests for:

• mention parsing with non-breaking space and newline
• /cc slash command normalization
• slash command event dispatch into the platform message handler
• slash-prefixed alias resolution

Verification run:

• go test -count=1 ./platform/slack ./core
• go test -count=1 ./...

Backward Compatibility

Existing Slack usage continues to work:

• @cc_connect /model
• DM plain-text commands like /model

This PR only adds a native Slack command path via /cc.

[chenhg5]

Thanks for this PR! Overall it looks great — it's backward compatible and adds a clean way to use cc-connect commands in Slack.

A few suggestions:

---

1. Make the gateway command configurable

Currently /cc is hardcoded. It would be better to make it configurable so users can choose their own gateway command name:

[[projects.platforms]]
  type = "slack"
  [projects.platforms.options]
    gateway_command = "cc"  # default: "cc", user can change to "ai", "bot", etc.

This way:

• Users can pick a command name that fits their workflow
• Avoids conflicts if /cc is already used by another Slack app

---

2. Future enhancement: prefix-based commands for better autocomplete

The current gateway approach (/cc model) only supports static usage hints.

A potential future enhancement would be a prefix mode where each command is registered individually (/cc_model, /cc_help, /cc_new, etc.). This would give users Slack's native autocomplete dropdown when they type /cc_.

This is more complex (requires registering N commands, handling dynamic skills/aliases), so it can be a follow-up PR. But worth considering for better UX.

---

Once the gateway_command config option is added, I'm happy to merge this. Thanks!