fix(slack): collect thread attachments across recent messages instead of only the newest

[shiminshen]

Description

Fixes #705.

collectInboundFileParts scans thread.recentMessages backwards, but the loop body ends with an unconditional return on the first non-bot message it inspects — the loop never advances. When a user posts an image in a thread and then mentions the bot in a text-only follow-up, the newest non-bot message has no files, so everything earlier is silently dropped and the model sees zero file parts. This is the exact case the function's own doc comment says the lookback exists to cover ("a user dropped a file in the thread first, then mentioned the bot in a follow-up").

The fix collects attachments across non-bot messages, newest first, capped at 10 file parts to keep model input bounded on long threads. If you'd rather keep single-message semantics ("most recent message that has files") instead of bounded accumulation, I'm happy to adjust — flagged the same question in #705.

We've been running this change in production for an internal Slack knowledge agent (as a patch-package patch of the dist) since eve 0.17.

How did you test your changes?

• pnpm run test:unit in packages/eve — 445 files, 4683 passed (adds a regression test that fails on main, plus accumulation and cap cases)
• pnpm run typecheck — clean
• pnpm lint — clean
• Production use via the equivalent dist patch in our Slack deployment

PR Checklist

• I linked an issue with prior discussion confirming this change is wanted (Slack channel: images/files posted earlier in a thread are silently dropped — attachment lookback only ever inspects one message #705 — filed with a failing-test reproduction branch; no maintainer reply yet, the one open design question is flagged there and above)
• I ran the relevant checks from CONTRIBUTING.md
• I added tests and documentation where relevant (JSDoc updated; no public API change)
• I added a changeset if this touches the published eve package
• DCO sign-off passes for every commit (git commit --signoff)

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[shiminshen]

One note on the THREAD_LOOKBACK_MAX_FILES = 10 bound: it's intentionally a hardcoded backstop to keep this diff minimal. If you'd prefer it configurable, I'm happy to lift it into UploadPolicy (e.g. an optional maxFiles, defaulting to 10, alongside maxBytes) — either in this PR or as a follow-up, whichever you prefer.