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.

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[zimeg]

Retries now live upstream in @slack/webhook (node-slack-sdk branch eden/webhook-retries, off the WWFB eden/webhook-trigger WIP) rather than in the action.

• Both IncomingWebhook and WebhookTrigger now accept a retryConfig?: RetryOptions option (mirroring @slack/web-api's WebClient), defaulting to no retries (opt-in).
• The webhook package re-exports the same named policies (fiveRetriesInFiveMinutes, tenRetriesInAboutThirtyMinutes, rapidRetryPolicy).
• Retry conditions match transient failures: 429 + 5xx retry, other 4xx abort, network errors retry.

This PR now just passes retryConfig through to the SDK constructors and maps 0/5/10/RAPID to the exported policies — no action-side p-retry wrapper. Pre-merge gate is unchanged: swap the file: link for a published @slack/webhook that includes these retry policies.