Remove unused Phylum/Postman CI jobs; document workflows in .github/README.md

[shambupujar]

Summary

Phylum, Postman sync, and Snyk are no longer used/working in this repo, so this PR removes them, documents the remaining workflows, and fixes a real CI build breakage found while verifying this branch.

CI cleanup

• Removed the phylum-analyze job from ci.yml and deleted phylum-daily-analysis.yaml entirely (no Phylum access anymore).
• Deleted postman.yml entirely (Postman collection sync is not used).
• Removed the snyk-scan-deps-licences and snyk-scan-code jobs from ci.yml — both failed on every PR with ResourceNotFoundException fetching AWS_SECRET_NAME_SNYK (the secret no longer exists), and there's no Snyk integration to fix it for.
• Verified via grep that no job in ci.yml or docker.yml depends on any of the removed jobs via needs:.
• Added .github/README.md: which workflows run automatically vs. need manual workflow_dispatch, the release sequence, and a table of each job's external dependencies (secrets, AWS IAM roles/Secrets Manager paths, self-hosted runner requirements, GitHub environment gates, third-party services).

Dockerfile fix

While validating this branch's CI, found main is currently broken: apt-get install fails with exit 100 across three build stages because several exact-pinned Debian bookworm package versions have been superseded and removed from the mirror. Fixed:

• library-build-stage-base (backs core-rust's buildRustForDocker/make build-core — i.e. the "Build debian package" and "Test core-rust docker build" CI jobs): curl deb12u14 → deb12u15, libssl-dev deb12u1 → deb12u2.
• java-build-stage: libssl-dev deb12u1 → deb12u2. Also pinned openjdk-17-jre/-jdk-headless/-jre-headless alongside openjdk-17-jdk — apt's resolver was picking the newer bookworm-security build for the unpinned transitive deps and then failing to satisfy the older jdk's exact-version Depends.
• app-container: curl deb12u14 → deb12u15.

Testing

• Verified via grep that no job depends on any removed job via needs: — safe to remove without breaking dependency chains.
• Verified each fixed Dockerfile stage builds cleanly against the live bookworm mirror: docker build --target library-build-stage-base and docker build --target java-build-stage both succeed; the app-container package set was verified by isolating and running its exact apt-get install line.
• .github/README.md is documentation-only; no workflow behavior changes there.

Note: this does not fix the Rust-toolchain-too-old failure also present on main (iter_repeat_n/is_multiple_of compile errors under the pinned 1.81.0 toolchain) — that's a separate, unrelated breakage tracked outside this PR.

[github-actions]

Docker tags
docker.io/radixdlt/private-babylon-node:pr-1075
docker.io/radixdlt/private-babylon-node:3b9553a0a7
docker.io/radixdlt/private-babylon-node:sha-3b9553a