Export Makefile vars so .env reaches gradle (#335)

## Summary

`sinclude .env` loaded `NPR_API_KEY` and `WRITE_BUCKET` as Make
variables but did not export them to recipe subprocesses, so `./gradlew
releasePlugin` invoked via `make release` failed with "Registry API key
must be configured...". Adding a bare `export` directive exports all
Make variables to children.

## Test plan

- [x] Verified `make release` previously failed at
`releasePluginToRegistry` with NPR_API_KEY missing
- [x] Verified `set -a; . ./.env; ./gradlew releasePlugin` succeeds
(used to actually publish 1.0.1)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- greptile_comment -->

<h3>Greptile Summary</h3>

This PR fixes `make release` failing at `releasePluginToRegistry` by
adding a bare `export` directive after `sinclude .env`, which causes GNU
Make to forward all Make variables (including `NPR_API_KEY` loaded from
`.env`) into the environment of recipe subprocesses like `./gradlew
releasePlugin`. The fix correctly solves the stated problem, though
exporting every variable is broader than the minimum needed.

<h3>Confidence Score: 4/5</h3>

Safe to merge — the fix is correct and the only finding is a style-level
concern about export scope.

Single P2 finding (bare `export` is broader than necessary); no logic
errors or security issues present.

No files require special attention.

<h3>Important Files Changed</h3>

| Filename | Overview |
|----------|----------|
| Makefile | Adds bare `export` directive so variables loaded from
`.env` via `sinclude` are forwarded to recipe subprocesses (e.g.
`./gradlew releasePlugin`); fix is correct but exports all Make
variables globally rather than only the needed credentials. |

</details>

<h3>Sequence Diagram</h3>

```mermaid
sequenceDiagram
    participant Dev as Developer
    participant Make as GNU Make
    participant Env as .env file
    participant Gradle as ./gradlew

    Dev->>Make: make release
    Make->>Env: sinclude .env (loads NPR_API_KEY, WRITE_BUCKET)
    Note over Make: export (all vars → subprocess env)
    Make->>Make: run `tag` prerequisite
    Make->>Gradle: releasePlugin (env contains NPR_API_KEY)
    Gradle-->>Dev: Plugin published ✓
```

<sub>Reviews (1): Last reviewed commit: ["Export Makefile variables so
.env
propag..."](c988438)
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---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: drernie

Makefile

@@ -1,6 +1,12 @@
 sinclude .env # create from example.env
+# Export to subprocesses (gradle, nextflow). Only export WRITE_BUCKET when
+# .env actually sets it; the integration tests use env.WRITE_BUCKET == null
+# as their @IgnoreIf signal, so a placeholder default would unmask them in CI.
+export NPR_API_KEY
+ifneq ($(strip $(WRITE_BUCKET)),)
+export WRITE_BUCKET
+endif
 PROJECT ?= nf-quilt
-WRITE_BUCKET ?= write-bucket-not-set
 FRAGMENT ?= &path=.
 QUERY ?= ?Name=$(USER)&Owner=Kevin+Moore&Date=2023-03-07&Type=CRISPR&Notebook+URL=http%3A%2F%2Fexample.com
 VERSION ?= $(shell grep "^version" build.gradle | head -1 | awk -F"'" '{ print $$2 }')