feat: post webhooks via the @slack/webhook SDK with action-side retries

[zimeg]

Summary

Converts both webhook techniques to the @slack/webhook SDK and removes axios / axios-retry as direct dependencies of the action:

• incoming-webhook → IncomingWebhook class
• webhook-trigger → WebhookTrigger class

Retry behavior (0 / 5 / 10 / RAPID) and HTTPS proxy support are preserved, so this is not a regression. Since the webhook SDK classes don't accept a retryConfig (unlike WebClient), retries run in the action via p-retry, reusing @slack/web-api's named retry policies. Non-retryable responses (4xx other than 429) abort immediately; request errors and 5xx/429 are retried.

src/config.js no longer owns an axios client or builds the webhook User-Agent — the SDKs set their own. The action still registers app metadata with @slack/web-api.

⚠️ Before merge

• @slack/webhook is pinned to a local file:../node-slack-sdk/packages/webhook path for development. It MUST be swapped to a published version (one that exports WebhookTrigger) before this merges.

Notes

• No @slack/web-api version bump in this PR; client.js (the API-method technique) is unchanged and still uses @slack/web-api@7 (axios transitively).
• dist/ is rebuilt as part of release, not committed here.

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

🤖 Generated with Claude Code

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