feat: add Slack markdown/text file support for input and output

[electronicBlacksmith]

Summary

• Inbound: Users can send .md/.txt file uploads in Slack, which are read into memory and injected as prompt text (no disk write)
• Outbound: Responses exceeding Slack's block size limit are uploaded as a .md file with a summary message instead of being chunked across multiple messages
• Graceful fallback to chunked messages if file upload fails (missing files:write scope, API error, etc.)

Changed files

File	Change
src/channels/types.ts	Added "text" to attachment type union, textContent field
src/channels/slack-files.ts	isTextFile(), hasNulBytes(), uploadSlackFile(), text file download support
src/channels/slack-formatter.ts	generateSummary() for file upload summaries
src/channels/slack.ts	extractTextFileContent(), inbound text extraction, outbound file upload in send()/postToChannel()/updateWithFeedback()
src/index.ts	Updated skipped-files message
Test files	30+ new test cases across all three test files

Test plan

• bun run typecheck passes
• bun run lint passes
• 933/934 tests pass (1 pre-existing flaky failure in events.test.ts, reproduces on main)
• Manual: send a .md file to Phantom in Slack, verify content is used as prompt
• Manual: trigger a long response (>2900 chars), verify summary message + .md file attachment
• Verify Slack app has files:write scope for outbound uploads

[electronicBlacksmith]

Pushed two follow-up commits fixing the loop final-notice issues in this feature's file-upload path:

8b3d546 — fix(loop): preserve full state body in final-notice upload and drop code fence

Two bugs in how the loop's final notice uses the new file-upload fallback:

1. extractStateSummary in src/loop/notifications.ts pre-truncated the state body at 3500 chars before postToChannel's upload branch ever ran, so the uploaded response.md only ever contained the first 3500 chars plus "…(truncated)", not the full log. Moved the cap off the extractor (which now returns the full frontmatter-stripped body) and onto a new optional 4th arg inlineFallbackMaxChars on postToChannel. Upload path stays verbatim; cap only applies to the chunked fallback.
2. postFinalNotice wrapped the summary in triple-backticks before posting, so when the upload path was unavailable and splitMessage chunked the fallback, every middle chunk rendered as an unterminated code block in Slack. Dropped the outer fence and restructured to post a short header and then the body as two sequential postToChannel calls.

Also added files:write to slack-app-manifest.yaml under oauth_config.scopes.bot so fresh installs pick up the scope the upload branch needs (the running workspace was granted it manually).

2f1946f — fix(slack): validate fallback cap and tighten regression tests

Follow-ups from a dual code review (Sonnet 4.6 + Copilot GPT-5):

• inlineFallbackMaxChars is now guarded against negative, NaN, and non-finite values. A negative cap would have made text.slice(0, -1) post "all but the last char" plus a truncation notice, defeating the cap. No current caller passes a bad value, but the validation removes the footgun.
• The "even number of triple-backticks per chunk" regression test in slack.test.ts was vacuous — it used plain-prose input with zero backticks, so 0 % 2 === 0 was always true regardless of the code under test. Replaced with an explicit not.toContain("\``")` assertion that directly guards against the removed outer-fence wrapper being reintroduced. Added a negative-cap test too.
• Documented in a comment that splitMessage is not code-fence aware, so a bounded body containing a long fenced block can still produce half-open fences in the chunked fallback. Fixing that is out of scope — the upload path sidesteps it entirely, which is the intended mitigation.

Verification

• bun run typecheck — clean
• bun run lint — clean
• bun test — 1040 pass / 0 fail (was 1034 before these commits)

Tests added/updated

• src/loop/__tests__/notifications.test.ts: updated "posts state.md body" to expect two postToChannel calls (header + body), assert the body has no code fence, frontmatter is stripped, and the 4th arg is 3500. Updated the status-fallback and multi-status tests for the new call count. Replaced the old "truncates very long summaries" test with a regression: 5000-char body reaches the body call with length ≥ 5000 and no truncation marker.
• src/channels/__tests__/slack.test.ts: new describe("SlackChannel postToChannel") block with four tests — full body reaches upload branch, cap applied on fallback with notice, chunked fallback with no cap doesn't introduce fences, negative cap is silently ignored.