Add scheduled proactive Agno Slack posts every 30 minutes

[ashpreetbedi]

Summary

• add a new tasks/proactive_agno_post.py scheduled task that builds short Agno updates from merged PRs, open PRs, local commits, or repo file spotlights
• register a 30-minute schedule on startup and add a manual POST /proactive-agno-post trigger endpoint
• add env/config/docs updates for channel C09GL0WK0SU, optional thread replies via 1775243740.375219, and lightweight per-bucket dedupe state

Validation

• python -m pytest /repos/coda/worktrees/proactive-agno-post/tests/test_proactive_agno_post.py
• python -m compileall /repos/coda/worktrees/proactive-agno-post/app /repos/coda/worktrees/proactive-agno-post/tasks /repos/coda/worktrees/proactive-agno-post/tests

Rollout notes

• Railway is configured with 3 replicas, so scheduled execution should be monitored for duplicate posts across processes even with the per-bucket dedupe file
• ship with PROACTIVE_POST_ENABLED=false, manually test POST /proactive-agno-post, then enable the schedule in the target thread/channel
• if the configured thread_ts becomes invalid, the task currently logs and falls back to stdout rather than silently posting elsewhere