fix(anthropic): honor CLAUDE_CONFIG_DIR when locating Claude Code credentials

[griffinwork40]

Symptom

A Linux/Windows user who sets CLAUDE_CONFIG_DIR to relocate Claude
Code's config directory gets silently switched from subscription/plan
billing to prepaid API credits when using hermes-agent as an Anthropic
provider. agent/anthropic_adapter.py looked for Claude Code's OAuth
credentials at a hardcoded ~/.claude/.credentials.json, never checking
CLAUDE_CONFIG_DIR. When that file isn't found at the default path,
resolve_anthropic_token() falls through its priority chain all the way
to the ANTHROPIC_API_KEY branch — trading Bearer-auth OAuth (plan
allowance) for x-api-key auth (metered credits), with all of the
diagnostic breadcrumbs at logger.debug. The credential refresh
write-back path had a second, worse instance of the same bug: on
refresh it wrote a stale duplicate credentials file to the default path
— a file Claude Code itself never reads.

Docs justification

Per Anthropic's official Claude Code docs
(https://code.claude.com/docs/en/authentication):

On Linux, credentials are stored in ~/.claude/.credentials.json. On
Windows, credentials are stored in
%USERPROFILE%\.claude\.credentials.json. If you've set the
CLAUDE_CONFIG_DIR environment variable on Linux or Windows, the
.credentials.json file lives under that directory instead.

CLAUDE_CONFIG_DIR was honored nowhere in this repo prior to this PR
(repo-wide grep returned 0 hits).

Fix

Added one module-level helper, _claude_code_credentials_path(), and
used it at both call sites that previously inlined the hardcoded path:

• _read_claude_code_credentials_from_file()
• _write_claude_code_credentials()

Behavior:

• If CLAUDE_CONFIG_DIR is set and non-empty after .strip(), resolves
  to <that dir>/.credentials.json.
• Empty/whitespace-only is treated as unset (matches the existing
  os.getenv(...).strip() convention already used in
  resolve_anthropic_token()).
• Expands ~ and $VAR via os.path.expanduser + os.path.expandvars
  (covers CLAUDE_CONFIG_DIR=~/foo).
• Resolved fresh on every call — never cached at import time — so
  processes that set/change the env var after import pick it up.

Platform-unconditional by design (no Darwin/Windows gate): when the
var is unset, the helper's output is byte-identical to the prior
hardcoded expression, so this is strictly additive. A platform check
would only add a branch with no behavioral benefit — macOS users who
never set CLAUDE_CONFIG_DIR get the same default path either way.

Relationship to #75146

Independent of, and does not conflict with, #75146 — that PR only
touches _read_claude_code_credentials_from_keychain (the macOS
Keychain path). This PR touches only the file read
(_read_claude_code_credentials_from_file) and the write-back
(_write_claude_code_credentials). Neither function nor line range
overlaps.

Testing

New file tests/agent/test_anthropic_claude_config_dir.py (14 tests,
real imports, no mocking of the module under test, tmp_path +
monkeypatch, invariant-based assertions):

• read path honors CLAUDE_CONFIG_DIR
• read path falls back to the default when unset
• empty/whitespace-only value falls back to the default (parametrized)
• ~ and $VAR expansion in the configured value
• env var re-read at call time, not cached
• write-back lands in the CLAUDE_CONFIG_DIR location, including a
  write → read round trip through the real read path
• existing fields in the credentials file are preserved when writing
  under CLAUDE_CONFIG_DIR

Verification

export HERMES_PYTHON=<repo>/.venv/bin/python
scripts/run_tests.sh tests/agent/test_anthropic_claude_config_dir.py   # 16 passed, 0 failed
scripts/run_tests.sh tests/agent/test_anthropic_adapter.py             # 84 passed, 0 failed (no regressions)
ruff check .                                                            # All checks passed
$HERMES_PYTHON scripts/check-windows-footguns.py --all                 # 0 footguns (893 files scanned)

Confirmed the worktree's interpreter imports the worktree's own code
(not the main checkout) via python -c "import agent.anthropic_adapter as m; print(m.__file__)" before running any gate.

Scope

Minimal and surgical — one new helper, two call sites updated to use it,
no changes to the atomic-write / 0600 / os.replace logic in
_write_claude_code_credentials, no refactor of the surrounding file.