Update documentation to match the current environment variables

[existemi]

Description

Bring the documentation back in line with the code.

• Correct the GitHub Enterprise Server section, which instructed setting GITHUB_API_URL. That variable is now ignored, and following the old instructions produces a warning on every startup
• Mark GITHUB_API_URL as deprecated and ignored in the README and the example .env
• Document seven environment variables that appeared in neither the README nor the example .env: GITHUB_RUNNER_VERSION, MAX_RUNNERS, ORKA_NAMESPACE, RUNNER_DEREGISTRATION_TIMEOUT, and RUNNER_DEREGISTRATION_POLL_INTERVAL, plus ENABLE_RECONCILIATION and VM_TRACKER_INTERVAL which were only in the example .env, and ORKA_VM_USERNAME and ORKA_VM_PASSWORD which were only in the README
• Clarify that GITHUB_TOKEN is always a github.com token, even on GHES, that it is used only to look up the latest runner release, and that it is unrelated to GITHUB_PAT. Note the 60 requests per hour unauthenticated limit and the 403 startup failure that follows from exceeding it
• Note that GITHUB_RUNNER_VERSION takes a bare version with no leading v, and that setting it removes the startup dependency on api.github.com entirely
• Add a note to the GitHub App setup guide that App authentication covers repository and organization runners only, linking to the enterprise section

Every environment variable defined in pkg/env/constants.go now appears in both the README and the example .env.

[ispasov]

nit — examples/.env: A few of the new keys are set to live values rather than commented placeholders — GITHUB_RUNNER_VERSION="2.336.0", ORKA_VM_USERNAME/PASSWORD="admin", and MAX_RUNNERS=9000. Since this file is meant to be copied as a starting point, anyone beginning from it silently pins the runner to 2.336.0 and inherits these as active config. Suggest commenting them out (like the other optionals) so the documented default is what actually takes effect unless the user opts in. Everything else reads accurately and matches the code.

[ispasov]

LGTM — docs read accurately and match the code (the GITHUB_API_URL deprecation, the "GITHUB_TOKEN is always a github.com token even on GHES" clarification, and the derived-API-URL behavior). Approving; the only thing outstanding is the non-blocking examples/.env nit (a few new keys set to live values rather than commented placeholders) — feel free to fold that in or ignore.

[existemi]

nit — examples/.env: A few of the new keys are set to live values rather than commented placeholders — GITHUB_RUNNER_VERSION="2.336.0", ORKA_VM_USERNAME/PASSWORD="admin", and MAX_RUNNERS=9000. Since this file is meant to be copied as a starting point, anyone beginning from it silently pins the runner to 2.336.0 and inherits these as active config. Suggest commenting them out (like the other optionals) so the documented default is what actually takes effect unless the user opts in. Everything else reads accurately and matches the code.

Good points. I have gone ahead and commented out everything that is default with the examples defaulting to the default value as well