feat(github): serve multiple GitHub owners, each with its own auth

[luthermonson]

Summary

ephemerd could only serve one GitHub owner — a single [github] block with one owner + one credential. This lets a single daemon serve multiple owners at once, each with its own auth (e.g. an org via a GitHub App and a personal account via a PAT), so one runner can process jobs across accounts.

[github]                 # primary: an org via GitHub App
owner = "ephpm"
app_id = 3290288
installation_id = 121756633
private_key_path = "/etc/ephemerd/app.pem"
repos = ["ephemerd"]

[[github_extra]]         # additional owner via a PAT
owner = "luthermonson"
token = "ghp_..."        # (or GITHUB_TOKEN for the primary)
repos = ["card-cannon", "better"]

How

The scheduler already handles a list of providers, and each GitHub Client is fully self-contained per owner — so this needs no owner-routing surgery inside the client:

• config: GitHubExtra []GitHubConfig + GitHubTargets() (primary [github] then [[github_extra]]) + per-target validation. Single [github] is unchanged.
• initProviders: loop GitHubTargets(), building one provider per target (App or PAT). Each polls/registers only its owner's repos and registers JIT runners with its own credential.
• webhook mode: each provider gets a stable, unique path — normally /webhook/github, disambiguated to /webhook/github/<owner> only when multiple GitHub targets exist. Single-target deployments keep the plain path (no change on upgrade).

Testing

• New pkg/config tests for GitHubTargets() + multi-target validation; pkg/config, pkg/github, pkg/providers/github, pkg/scheduler all compile.
• Verified live (poll mode): one config with ephpm (App) + luthermonson (PAT) authenticated to both; a self-hosted job queued in the private luthermonson/card-cannon was discovered only under the luthermonson target, while the ephpm (App) target correctly ignored it.

Notes

• Job execution wasn't exercised here (needs the full runtime on a provisioned runner); this PR is the auth + per-owner discovery/routing layer.
• Follow-up: the mayfly side renders [[github_extra]] into runner configs.