docs: add CONTRIBUTING with the canonical-on-Gitea PR flow

Moves developer, testing and release instructions out of the README,
minus the fixture-stripping advice, which was wrong.

Explains that a GitHub PR closes itself as merged once the mirror syncs,
because the merge preserves the contributor's SHAs -- so a PR closing
without a visible Merge click reads as success rather than rejection.

Documents the cog-api dependency: its CI clones this package at
USCOGDATA_REF, defaulting to main with no pin, so anything merged here
reaches the API's next build. Includes the commands to run its suite
against a branch first.

.Rbuildignore

@@ -14,3 +14,4 @@
 ^\.gitea$
 ^CLAUDE\.md$
 ^\.superpowers$
+^CONTRIBUTING\.md$

CONTRIBUTING.md

@@ -0,0 +1,105 @@
+# Contributing to uscogdata
+
+Thanks for reading this — a package like this gets better mostly through people
+noticing that a number looks wrong.
+
+## Where the code lives
+
+Development happens on **Gitea**, at
+`gitea.civilytics.org/Civilytics/uscogdata`. The repository at
+`github.com/civilytics/uscogdata` is a **mirror** that accepts issues and pull
+requests.
+
+## What happens to a GitHub pull request
+
+Open it normally. Behind the scenes it is fetched and landed on the canonical
+Gitea repository, then syncs back:
+
+```sh
+git fetch github refs/pull/42/head:pr-42
+git switch main && git merge --no-ff pr-42
+git push origin main            # Gitea -> mirror -> GitHub
+```
+
+Because the merge preserves your commits at their original SHAs, **GitHub marks
+your PR merged on its own** as soon as the mirror syncs. So:
+
+> If your pull request closes as "Merged" without anyone visibly clicking
+> Merge, that is the normal, successful outcome — not a rejection.
+
+Substantial contributions get a `ctb` entry in `DESCRIPTION`, which surfaces in
+`citation("uscogdata")`.
+
+There is no CLA and no DCO sign-off requirement.
+
+## Running the tests
+
+```r
+devtools::test()   # bundled fixture; no network, no credentials
+```
+
+`tests/testthat/setup.R` points `USCOGDATA_URL` at
+`inst/extdata/fixture_corpus/` automatically — a four-year slice (2011, 2012,
+2019, 2020) covering all 50 states. That is the whole data setup.
+
+## Testing against the live corpus
+
+```sh
+USCOGDATA_LIVE_TEST=true Rscript -e 'devtools::test(filter = "live-corpus")'
+```
+
+This is worth understanding rather than skipping. Until 0.3.0 the package
+**could not read a remote corpus at all** — the partitioned view used a glob,
+and DuckDB cannot expand a glob over generic HTTP. It went unnoticed for months
+because every test path used a local corpus (the bundled fixture), and so did
+the production API (a host mount). Nothing exercised the package the way a new
+user does.
+
+`test-live-corpus.R` is the only test that runs with no `USCOGDATA_URL`, no
+option, and no fixture. If you change anything touching view registration,
+manifest handling, or configuration, run it.
+
+## Do not exclude the fixture from the build
+
+There is a temptation to add `^inst/extdata/fixture_corpus$` to
+`.Rbuildignore` because 15 MB feels large for a package. Don't:
+
+- `vignette("total-spending")` reads from it and would fail to build.
+- `R CMD check` on r-universe and GitHub Actions would have no corpus, so the
+  suite could not run without credentials.
+
+This package is not going to CRAN, so its 5 MB guidance does not apply. A
+package-size NOTE in `R CMD check` is expected and acceptable.
+
+## Downstream consumers
+
+`cog-api` depends on this package and its CI clones uscogdata at
+`USCOGDATA_REF`, **defaulting to `main`**. There is no pin. Anything merged
+here reaches the API's next build, so before merging a change to the reader,
+run the API suite against your branch:
+
+```sh
+Rscript -e "remotes::install_local('/path/to/uscogdata', upgrade = 'never')"
+cd /path/to/cog-api/api/tests/testthat
+Rscript -e 'testthat::test_dir(".", stop_on_failure = TRUE)'
+```
+
+The API calls only exported verbs, so internal refactors are usually safe —
+but "usually" is not a release gate.
+
+## Release checklist
+
+1. `devtools::test()` — green against the bundled fixture, offline.
+2. `USCOGDATA_LIVE_TEST=true devtools::test()` — green against the live corpus.
+3. cog-api suite green against this branch (above).
+4. `devtools::check(args = "--as-cran")` — 0 errors, 0 warnings.
+5. `pkgdown::build_site()` completes.
+6. Vignettes resolve from an installed copy:
+   `vignette("total-spending", package = "uscogdata")`.
+7. **Cold-start check**: on a machine that has never had this package,
+   install it and run the README quickstart verbatim with no environment
+   variables set. This is the only check that catches a
+   corpus-unreachable defect, and its absence is why 0.3.0 needed fixing.
+8. Bump `Version` and add a `NEWS.md` section.
+9. Tag, then update the r-universe registry pin at
+   `github.com/civilytics/civilytics.r-universe.dev`.