fix: harden V2 beta release and bridge capacity

[CasualDeveloper]

Summary

Follow-up to #31 and #32. This restores the release and transport safeguards identified after the V2 work moved to the upstream beta branch.

Changes

• Restore automatic beta CI with the complete npm run verify gate.
• Replace the stable-version release job with a beta-only workflow that validates and publishes the exact reviewed prerelease artifact.
• Make release dry runs write-free and default them to enabled.
• Enforce the configured bridge worker limit instead of spawning untracked overflow workers.
• Release or remove worker capacity before notifying close callbacks, and isolate callback failures from the worker reader.
• Validate bridge pool bounds and keep repeated warmup within maxSize.
• Return retryable HTTP 503 responses when the preserved V1 proxy reaches bridge capacity.
• Restore the documented context-window and output-token environment overrides with positive-integer validation.
• Add shared bridge/configuration regressions to the V2 test gate and update the V1 smoke coverage for bounded capacity.

Verification

• npm run verify
• V1 smoke suite passes, including real proxy saturation and capacity reuse.
• 40 V2/shared tests pass.
• Typecheck and build pass.
• Packed dual V1/V2 package validation passes.
• OpenCode V2 loader smoke passes.

Refs #31.