test: make config default tests hermetic against host config

[joestump]

Problem

go test ./internal/config/ fails on any developer machine that has a real
msgbrowse config file installed:

--- FAIL: TestLoadDefaults
    config_test.go:29: LLM.ChatModel = "gpt-oss:120b", want local-first default local-chat
    config_test.go:32: LLM.EmbedModel = "text-embedding-3-large", want local-first default local-embed

config.Load("") adds real host search paths — ., $HOME/.config/msgbrowse,
and os.UserConfigDir()/msgbrowse (macOS: ~/Library/Application Support/msgbrowse)
— so ReadInConfig() picks up the developer's actual config.yaml, whose
chat_model/embed_model override the code defaults the test asserts. The test
therefore passes only on machines with no installed config (e.g. CI).

Fix

Add a loadHermetic(t) helper that points HOME and XDG_CONFIG_HOME at an
empty t.TempDir() before Load(""), so the file search finds nothing and the
built-in defaults are exercised regardless of what's installed on the host. All
three Load("") call sites (TestLoadDefaults, TestEnvOverride,
TestValidate) go through it.

This mirrors the hermetic-HOME approach used for the setup detection tests in
7cbbe3a (#214). save_test.go already uses Load(path) with an explicit temp
path, so it needed no change.

Verification

• go test ./internal/config/ and go vet ./internal/config/ — clean.
• Reproduced the exact reported failure to confirm the fix addresses the cause,
  not just the symptom: with the HOME/XDG overrides temporarily removed and
  HOME pointed at a fake tree containing a config.yaml setting
  chat_model: gpt-oss:120b / embed_model: text-embedding-3-large,
  TestLoadDefaults failed with the identical assertions from the bug report;
  the hermetic version passes even with that same host config present.

Test-only change; independent of the onboarding MOV-warning fix on
claude/mov-converter-errors-52b465.

🤖 Generated with Claude Code

🤖 Posted on behalf of @joestump by Claude.

[joestump]

Superseded by the canonical Gitea PR: https://gitea.stump.rocks/stump.wtf/msgbrowse/pulls/247

The head commit was pushed to gitea.stump.rocks/stump.wtf/msgbrowse unchanged and the PR reopened there against main. Nothing from this branch is lost — the commit SHA is identical.

msgbrowse now develops on Gitea; stump.wtf/msgbrowse is the source of truth, push-mirrored to https://github.com/stump-wtf/msgbrowse. This personal repo is being archived.

🤖 Posted on behalf of @joestump by Claude.