Remove retired custom deployment target from Caddy rollout

[simple-analytics-ai]

Summary

Closes #118

Removes the retired custom deployment destination (the nginx server at app@external.simpleanalytics.com) from the deployment CLI, leaving cdn and caddy as the only targets. The Destination union, remoteTarget/RemoteTarget, uploadCustomFile, and the custom-server inspection and verification branches are deleted; readCustomFiles becomes readCaddyFiles against CADDY_HOST/CADDY_ROOT. addServerPair collapses into addCaddyPair, which always applies the caddy-ssi transform.

The destination prompt now offers CDN and Caddy with both selected by default, the proxy variant is offered when Caddy is selected, --caddy-only is unchanged, and the obsolete nginx reminder is replaced by a plain SRI documentation reminder. Caddy keeps consuming the existing dist/**/custom/ artifacts, so no generated script or SRI bytes change.

This targets PR #95 (caddy): merge this cleanup into #95 first, then merge #95 into main.

Changes:

• Remove retired custom deployment target

Security implications

Has security impact - described as: changes production deployment routing by removing a retired destination and making Caddy a default target. Remote-path validation, shell quoting, Go-template escaping of proxy query values, --ignore-existing protection for immutable SRI artifacts, and post-upload content verification are all preserved.

Testing

• node --test test/deploy.test.mts — 17/17 pass.
• tsc (project tsconfig.json, --noEmit) — clean, no errors.
• No production deployment performed.

Checklist

• Linked to an issue
• Tested
• Asked for a review

[github-actions]

README.md:41 still documents the deleted target: "Custom-domain reads and writes require SSH access to app@external.simpleanalytics.com", which no longer exists after this PR, so a deployer provisions access to the wrong host. Update that sentence to the remaining SSH target (user@esapp05.simpleanalytics.com, the CADDY_HOST in deploy.mts).

[github-actions]

Claude review checkpoint

Reviewed commit bd186eec2f73957d378471a76b6729021c74a60a. This is used to keep later automated reviews focused on changes Claude has not checked yet.

[github-actions]

SDLC label: change: needs review because Changes production deployment routing by deleting a deployment destination and its upload/verification paths, and makes Caddy a default-selected target for customer-facing script releases