Remove or wire up the dead GithubEnterprise / GetGHE code path

[dolph]

Summary

config.go defines GithubEnterprise, httpclient.go defines GetGHE and GHEKey and GHEKeysToKeys. None are reachable from cmd.go:Run's code path — the YAML config only deserializes Sources (slice of Source, which is just {URL string}), and only client.GetURL is ever called. The GHE types and methods are dead code.

Why this matters

Three concrete costs of leaving it in:

1. Latent security risk — issue httpclient.go: cache key is the URL only, so different auth tokens for the same URL share cached data #4 is specifically about a cache-key bug in GetGHE (Authorization header isn't part of the cache key). That issue is rated Medium because it's "latent until GHE is wired up". If GHE is never going to be wired up, httpclient.go: cache key is the URL only, so different auth tokens for the same URL share cached data #4's risk is permanent (because the latent code keeps existing in the binary), and if GHE is going to be wired up, httpclient.go: cache key is the URL only, so different auth tokens for the same URL share cached data #4 becomes Critical the day that lands. Either way, the dead code makes the security posture less clear.

2. Maintenance load on every PR — every time someone touches httpclient.go (PR Stop killing the process on HTTP fetch errors (closes #2) #10 did, this issue's neighbor PRs likely will), they have to read and reason about the GHE branch, even though it's unreachable. PR Stop killing the process on HTTP fetch errors (closes #2) #10 specifically had to make the same log.Fatal-removal changes in GetGHE that it made in GetURL, doubling the diff for no runtime benefit.

3. Misleading code surface — a maintainer reading the code reasonably believes ussher supports GitHub Enterprise. It doesn't, today.

Approach — two options, pick one

A. Remove the dead code

• Delete GithubEnterprise from config.go.
• Delete GHEKey, GHEKeysToKeys, GetGHE from httpclient.go.
• Delete the encoding/json import (currently only used by the GHE path).
• Note in the PR that httpclient.go: cache key is the URL only, so different auth tokens for the same URL share cached data #4 is closed by removal — there's no longer code with the Authorization-cache-key issue.
• ~50 lines net deletion.

B. Wire it up

• Add a github_enterprise field (or list of GHE entries) to Config.
• Add the resolver in LoadConfigByPath so YAML deserializes them.
• In cmd.go:Run, iterate over the GHE entries alongside Sources and dispatch to GetGHE.
• Fix httpclient.go: cache key is the URL only, so different auth tokens for the same URL share cached data #4 in the same PR (cache key includes Authorization).
• Document in the README.
• ~80–120 lines net addition.

Option A is the lower-effort path and is the right call if the maintainer doesn't have a near-term plan for GHE adoption. Option B is right if there's a concrete user / use-case waiting on it.

A third option — leave it as-is — has a real ongoing cost (the maintenance + #4 ambiguity above) so I'd avoid it.

Note on history

The pre-v1.0.0 commit a81fede "remove mostly-unimplemented support for GHE" already attempted a partial removal. The struct definition and the methods survived that pass. Worth understanding what that commit intentionally kept (if anything) before removing the rest.

Acceptance criteria (option A)

• No remaining reference to GithubEnterprise, GHEKey, GetGHE, GHEKeysToKeys in any non-test file.
• encoding/json import is removed from httpclient.go if nothing else needs it.
• TestGetGHE_UnreachableIsNotFatal is removed (and TestGetGHE_UnreachableIsNotFatal performs a real DNS lookup; flaky in offline CI #21 is closed by removal).
• httpclient.go: cache key is the URL only, so different auth tokens for the same URL share cached data #4 can be closed (or rescoped to "if GHE is reintroduced, it must include Authorization in the cache key").

Acceptance criteria (option B)

• YAML config can declare a GitHub Enterprise source with api_hostname, user, token.
• Run invokes GetGHE for each declared GHE source.
• httpclient.go: cache key is the URL only, so different auth tokens for the same URL share cached data #4 is fixed in the same PR (cache key includes a hash of the Authorization value).
• README documents the GHE config shape.

Files

• /home/user/ussher/config.go
• /home/user/ussher/httpclient.go
• /home/user/ussher/httpclient_test.go
• (option B) /home/user/ussher/cmd.go, /home/user/ussher/README.md