[codex] Align safe-agent bridge proof docs

[Atomics-hub]

What changed

• Updated the v0.2 alpha release notes to describe Slack/GitHub/email delivery dry-runs and Postgres push dry-run as store/notification handoff evidence.
• Added the safe-agent demo objective language for local Slack/GitHub/Postgres bridge dry-run proof.
• Updated the public readiness smoke checklist to expect notification payloads plus bridge dry-run reports.

Why

The code now requires bridge dry-run proof for both store/notification handoff and the top-level safe-agent demo objective. The release-facing docs needed to match that stronger evidence contract.

Validation

• cargo run --locked -- release-status --json | jq '{ready_for_alpha_rc, shipped: [.shipped_surfaces[].name], gates: [.verification_gates[].name]}'
• cargo run --locked -- release-ticket --out dist/release-ticket-doc-bridge-proof --force --json | jq '{ready, safe_agent: (.checks[] | select(.name=="objective: safe-agent demo")), store: (.checks[] | select(.name=="store/notification handoff"))}'
• rg -n 'bridge dry-run|delivery dry-runs|Postgres push dry-run|notification payloads and bridge dry-run reports|safe-agent demo objective' docs/v0.2-alpha-release-notes.md docs/public-readiness.md
• cargo fmt --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo run --locked -- release-audit