fix: support OpenClaw SecretRef tokens

[TeodoroRodrigo]

Summary

• accept OpenClaw Discord token values as either plaintext strings or SecretRef objects
• resolve env-backed and file-backed SecretRefs, including file providers configured under secrets.providers.filemain or secrets.filemain
• cover account-scoped Discord token refs with regression tests

Why

Discrawl 0.6.0 failed during discrawl init --from-openclaw ~/.openclaw/openclaw.json when OpenClaw stored channels.discord.token as an object pointing at a secrets file instead of as a plaintext string.

Validation

• go test ./internal/config
• go test ./...
• go test ./internal/cli -run 'Test(RuntimeInitSyncTailAndDoctor|RunInitWritesDiscoveredGuildConfig)' -count=1
• git diff --check

AI-assisted: yes. I understand the change: it only extends OpenClaw token loading compatibility and does not log or persist resolved secret values.

[steipete]

Landed via squash merge onto main.

• Gate: go test ./...; after changelog metadata follow-up: go test ./internal/config, git diff --check
• Land commit: 6986104
• Merge commit: 4cc6bb0

Thanks @TeodoroRodrigo!