docs: convert vignettes to .md for GitHub rendering

[mcmullarkey]

Summary

GitHub renders .Rmd files as raw source, so the 6 troubleshooting vignettes were unreadable on the repo. Converted them to plain Markdown (.md) so GitHub renders them as formatted pages.

• 6 vignettes converted (.Rmd → .md, old files deleted): faq, redcap-troubleshooting, google-troubleshooting, abs-troubleshooting, docker-troubleshooting, ci-troubleshooting
• Content preserved verbatim (prose, tables, headings, links) except: knitr/rmarkdown YAML frontmatter removed (plain # title kept), {r ... eval=FALSE} / {bash, eval=FALSE} chunk fences → plain fenced code blocks (display-only — plain Markdown cannot execute code), one stale prose parenthetical "(eval = FALSE in every chunk)" → "(plain Markdown, so GitHub renders it without running anything)"
• README Troubleshooting section: links + wording updated for .md
• abmdash-guide skill: vignettes link → vignettes/faq.md
• test-faq-verbatim.R: reads vignettes/faq.md; fenced-block extraction now detects code blocks by content (comment/assignment/Rscript/env-var lines) since .md has no {r} chunk headers; eval=FALSE chunk test replaced with a no-knitr-fences guard (hermeticity trivially satisfied by plain Markdown)
• DESCRIPTION: VignetteBuilder: knitr + Suggests: knitr removed (dead config with no .Rmd left; knitr stays installed via rmarkdown Imports, renv.lock untouched)

Verification

• Tests: devtools::test() → 490 PASS / 0 FAIL / 0 WARN (faq-verbatim context: 61 assertions, including all verbatim error-string + OKF-link checks)
• make lint (devtools::check()) before/after:
  • Before (main): 1 ERROR, 5 WARNINGs, 4 NOTEs
  • After: 1 ERROR, 6 WARNINGs, 5 NOTEs
  • Same 1 ERROR on both (pre-existing, NOT caused by this change): 3 test failures under R CMD check — repo_read("Makefile") can't open the file (Makefile isn't in the check tarball; the guard is designed for devtools::test(), not R CMD check) + 2 file.exists() failures in test-run-initial-function (extdata paths resolved against the check dir)
  • +1 WARNING +1 NOTE — the expected vignette issue, flagged for your decision:
    • WARNING "checking files in 'vignettes' ... Files in the 'vignettes' directory but no files in 'inst/doc'"
    • NOTE "checking package vignettes ... Package has 'vignettes' subdirectory but apparently no vignettes."
    • R CMD build tolerates the .md files (no ERROR, build completes); they're just not recognized as vignettes. Options: (a) accept the WARNING+NOTE as-is, (b) move .md docs to a docs-md/ folder outside vignettes/ (your suggested alternative — then update README/skill/test paths too), (c) exclude vignettes/ via .Rbuildignore. I did NOT pick — flagging per the task instructions.
• Stray .Rmd grep: zero live references outside git history. Remaining hits are only in .opencode/plans/repo-usability/ — historical committed plan/spec records for the completed feature (they document what existed at the time; not live docs, left untouched).

Deviations (reported honestly)

1. faq.md internal cross-links (Still stuck? section) updated .Rmd → .md — Task 1 said preserve links verbatim, but the .Rmd targets are deleted, so leaving them would 404. Links must follow the rename.
2. One prose parenthetical in faq.md updated (see Summary) — "eval = FALSE in every chunk" is false for a file with no chunks.
3. No changes to R/ source, workflows, docs/ (dashboard), or .Rbuildignore — check did not demand any.

Test plan

• Rscript -e 'devtools::test()' — 490 PASS / 0 FAIL
• make lint — before/after compared (see above), only the expected vignette WARNING+NOTE added
• Stray-.Rmd grep — zero live refs

ACs

N/A — fast-track, no linked issue.