docs: document dev image promotion policy

[weicao]

Summary

• document accepted immutable dev and release-candidate sources
• allow explicit stable targets across prerelease baselines
• add the v0.1.0-dev.20260822.3 to v0.2.0 promotion example

Scope

• Documentation only: docs/release.md
• Parent: fe3be28
• Signed head: 2d2b5ac
• Tree: e437ee1522c78492593aef23973fc8bf5219bde4
• No workflow dispatch or registry writes

Closes task #35.

[leon-ape]

Reviewed exact signed head 2d2b5ac. No blocking findings. The diff is limited to docs/release.md and accurately documents promotion of accepted immutable development or release-candidate images, an explicitly selected stable vX.Y.Z across prerelease bases when intentional, and v0.1.0-dev.20260822.3 -> v0.2.0 plus latest without rebuilding. Parent, tree, signature, and diff-check match the frozen review target. No workflow dispatch or registry write was performed.