fix: write README to crate dir for cargo package (#175)

## Summary

- `release-hook.sh` was writing README only to `../../README.md`
(workspace root)
- `Cargo.toml` has `readme = "README.md"` which cargo-release resolves
relative to the crate directory (`crates/cull-gmail/README.md`)
- cargo-release failed with "readme does not appear to exist"

Fix: write README to `README.md` (crate directory) first, then copy to
`../../README.md` for GitHub display.

## Test plan

- [ ] CI validation passes
- [ ] Release pipeline succeeds (README found at crate level)

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

crates/cull-gmail/release-hook.sh

@@ -1,11 +1,13 @@
 #!/bin/sh
 
 # Build an updated README
-cat ../../docs/readme/head.md > ../../README.md
+cat ../../docs/readme/head.md > README.md
 # shellcheck disable=SC2129
-cat ../../docs/main.md >> ../../README.md
-cat ../../docs/lib.md >> ../../README.md
-cat ../../docs/readme/tail.md >> ../../README.md
+cat ../../docs/main.md >> README.md
+cat ../../docs/lib.md >> README.md
+cat ../../docs/readme/tail.md >> README.md
+# Also update workspace root README for GitHub display
+cp README.md ../../README.md
 
 # Build Changelog
 gen-changelog generate \