Remove deprecated config fields (proxyUrl, sslKeyPemPath, sslCertPemPath) in 3.0

[dcoric]

Summary

Remove the deprecated top-level config fields proxyUrl, sslKeyPemPath, and sslCertPemPath from the schema, types, and config loader. These fields are already flagged deprecated: true in config.schema.json and have been superseded by upstreamProxy and tls.key / tls.cert respectively.

Why defer to 3.0

Removing these fields is a breaking change: after the upgrade, the app will fail to start unless users clean up their config files. That's not acceptable inside the 2.x line, so this work should land in the next major.

Context

• Discussed in feat(config): require merged top-level defaults #1514 (review thread with @jescalada).
• The removal was briefly included in feat(config): require merged top-level defaults #1514 as commit c48d0bf but reverted in 4234c16 to keep feat(config): require merged top-level defaults #1514 non-breaking.

Scope

• Drop proxyUrl, sslKeyPemPath, sslCertPemPath from config.schema.json.
• Regenerate src/config/generated/config.ts.
• Remove the legacy fallback logic in src/config/index.ts (mergeConfigurations, getProxyUrl, getTLSKeyPemPath, getTLSCertPemPath).
• Simplify FullGitProxyConfig to plain Required<GitProxyConfig> (drop OptionalTopLevelConfigKey).
• Update reference docs (website/docs/configuration/reference.mdx).
• Improve the config-load error message so users hitting a removed/unknown key get a clear "this field is no longer supported, use X" instead of the current misleading "expected boolean but got X" (per @jescalada's note in feat(config): require merged top-level defaults #1514).

Migration for users

Deprecated field	Replacement
proxyUrl	upstreamProxy
sslKeyPemPath	tls.key
sslCertPemPath	tls.cert