feat: modernize cleanup workflow and add Zulip

[chck]

Summary

• modernize synr to Go 1.27 with current Slack and YAML dependencies
• replace the legacy implementation with fail-closed domain, use-case, and adapter layers
• add preview-by-default cleanup for Slack, Chatwork, and Zulip; mutation requires --apply
• move credentials to environment variables and validate strict non-secret YAML configuration
• add race-tested provider fixtures, pre-commit checks, GitHub Actions, and GoReleaser v2 configuration
• document the new CLI, configuration, credentials, and provider-specific safety rules

Safety behavior

• list and evaluate every conversation before any mutation
• never leave protected, unknown-activity, malformed, or unsupported conversations
• stop at the first leave failure and report partial progress
• inspect complete Slack channel and history pagination, including thread replies
• protect Slack general, Chatwork direct/sticky/My Chat, and Zulip pinned channels

Breaking changes

• CLI is now synr scan --service <slack|chatwork|zulip>
• configuration uses ${XDG_CONFIG_HOME:-$HOME/.config}/synr/config.yaml
• credentials are read only from SYNR_* environment variables
• the old secrets file and legacy command flags are removed

Verification

• makers check (format, vet, race tests, build, govulncheck)
• makers cli-help
• makers modules
• makers test-preview-repeat
• makers goreleaser-check
• pre-commit run --all-files
• actionlint .github/workflows/ci.yml .github/workflows/release.yml
• git diff --check

No live service mutation or release was performed. Discord support remains deferred to #11.