feat(multi-env): add multi-environment deployment support

[aparajon]

Summary

Add support for running separate SchemaBot instances per environment (e.g., staging and production on isolated infrastructure). Each instance handles a subset of environments with its own GitHub App and database.

• allowed_environments config field scopes which environments an instance handles
• Per-environment aggregate check runs: SchemaBot (staging), SchemaBot (production)
• Config-driven prior-environment enforcement: owned environments checked via local DB, remote environments checked via GitHub Checks API
• Environment filtering in webhook handlers — commands for non-allowed environments are silently skipped
• Fully backwards compatible — single-deployment mode unchanged when allowed_environments is not set

# Staging instance
allowed_environments: [staging]
tern_deployments:
  default:
    staging: "tern-staging:9090"

# Production instance
allowed_environments: [production]
tern_deployments:
  default:
    production: "tern-production:9090"

The production instance enforces "staging before production" by checking GitHub for the SchemaBot (staging) check run from the staging instance — no cross-environment networking needed.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds multi-environment deployment support so separate SchemaBot instances can own different environments, publish environment-scoped aggregate checks, and coordinate prior-environment enforcement across instances.

Changes:

• Added allowed_environments config support and documentation for multi-instance deployments.
• Filtered webhook command handling and multi-env planning to only process environments owned by the current instance.
• Added GitHub-check-based prior-environment verification and environment-scoped aggregate check naming/tests.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
pkg/webhook/plan.go	Filters multi-environment plan execution to this instance’s allowed environments.
pkg/webhook/issue_comment.go	Skips env-targeted commands that belong to another instance.
pkg/webhook/check_runs.go	Adds per-environment aggregate names and remote prior-environment checks.
pkg/webhook/check_runs_test.go	Adds unit tests for new check-run and environment-filtering helpers.
pkg/github/client.go	Adds helper to find a check run by name on a commit.
pkg/api/config.go	Introduces allowed_environments config and lookup helper.
pkg/api/config_test.go	Tests new config behavior and YAML deserialization.
docs/configuration.md	Documents multi-environment deployment setup and behavior.

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