fix(src): reliably post and auto-pin the help walkthrough

[phorcys420]

What

Fixes two issues with the #help forum walkthrough:

1. Sometimes never posts on new threads. A forum post's ThreadCreate fires
   before the OP's starter message lands, and Discord rejects bot messages until
   it does (DiscordAPIError[40058]). The old channel.send had no error
   handling, so the rejection was swallowed and the walkthrough silently went
   missing (worse when the starter post has a large upload).
2. Never auto-pins. Pinning only happened after all three fields were
   answered, so the auto-sent message was left unpinned.

Changes

• sendWalkthrough() retries the send up to 5x (~2s apart, ~10s window),
  catching only 40058 and rethrowing anything else.
• pinQuietly() pins the message and swallows + logs failures.
• Walkthrough is pinned the moment it is posted, on both the auto-send and the
  /walkthrough fallback paths. The pin-on-completion block is removed.
• ThreadCreate now gates on newlyCreated so re-cached threads on gateway
  reconnect don't double-post.

Verification

bunx tsc --noEmit, bun format, and bun lint all clean. No test suite in
this repo.

Implementation plan

Context / root cause

• Auto-send is wired to ThreadCreate in src/events/walkthrough.ts, which
  calls doWalkthrough -> channel.send(...) with no error handling.
• Discord fires ThreadCreate for a forum post before the OP's starter message
  lands. Sending too early is rejected with DiscordAPIError[40058]. It is
  intermittent because the delay grows when the starter post has a large upload.
  The rejection is swallowed as an unhandled rejection, so the walkthrough
  silently never appears.
• Pinning only happened in handleSelection once all three fields were answered
  (values.length === steps.length).

Changes

src/events/walkthrough.ts

• Accept the second ThreadCreate arg and bail when not newly created.

src/commands/util/walkthrough.ts

• sleep, STARTER_NOT_READY = 40058, sendWalkthrough (retry with backoff),
  and pinQuietly helpers.
• Auto path: try/catch, send via retry, pin, log on give-up.
• Interaction path: pin the reply via fetchReply().
• Remove the pin-on-completion block in handleSelection.

No config, schema, command-registration, or dependency changes.

Out of scope

• Reworking the auto-send to trigger off the OP's first MessageCreate (cleaner
  but larger; retry covers the reported bug).
• Selector UI, log guides, or Linear bridge changes.

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