chore: migrate warden.toml to [[skills]] format

[gricha]

Summary

• Migrate from legacy [[triggers]] format to current [[skills]] format
• Add type = "local" triggers so skills also run via warden CLI (not just on PRs)
• No behavior change for PR workflow -- same skills, same paths, same actions

Before

[[triggers]]
name = "security-review"
event = "pull_request"
actions = ["opened", "synchronize", "reopened"]
skill = "security-review"
remote = "getsentry/skills"

[triggers.filters]
paths = ["src/**", "web/**", "mobile/**"]

After

[[skills]]
name = "security-review"
paths = ["src/**", "web/**", "mobile/**"]
remote = "getsentry/skills"

[[skills.triggers]]
type = "pull_request"
actions = ["opened", "synchronize", "reopened"]

[[skills.triggers]]
type = "local"

Test plan

• Warden GitHub Action runs on this PR and matches the same triggers
• warden HEAD from CLI picks up local triggers