fix(github): use api.github.com base URL for GitHub App auth (#1823)

[hugh-fai]

Summary

Fixes #1823 — the branch-planner returns 404 Not Found when listing pull requests under GitHub App authentication.

Root cause

In internal/git/provider/github_provider.go, Setup() builds the go-scm client for the App-auth path with the web host https://github.com instead of the API host https://api.github.com:

p.client, err = github.New(serverURL)   // serverURL == https://github.com for github.com

All subsequent REST calls (e.g. listing PRs) are then issued against https://github.com/... and 404. The token auth path avoids this because it goes through factory.NewClient → ensureGHEEndpoint, which resolves the correct API host.

Fix

Use the API host in the App path, mirroring the token path's ensureGHEEndpoint behaviour:

apiURL := "https://api.github.com"
if p.hostname != "github.com" {
    apiURL = serverURL + "/api/v3"
    transport.BaseURL = apiURL
}
p.client, err = github.New(apiURL)

• github.com: now uses https://api.github.com (was https://github.com).
• GHE: now uses <server>/api/v3 for both transport.BaseURL and github.New() (previously only transport.BaseURL was set, while the web host was still passed to New()).

Verification

Built a patched branch-planner image from this fix and ran it against a live cluster with a GitHub App-authenticated branch-planner. Before: every poll logged Not Found (404) when listing PRs. After: the branch-planner lists PRs successfully, discovers the open PR, and reconciles its Terraform planner object with zero 404s (0 error-level log lines across many poll cycles). The App-auth PR-listing path that #1823 reported is fixed.

Fixes #1823