refactor(common): extract resolve_data_dir helper from get_db_path

[geoffjay]

refactor(common): extract resolve_data_dir helper from get_db_path

refactor(common): extract resolve_data_dir helper from get_db_path (closes #722)

[geoffjay]

This change is part of the following stack:

• refactor(common): extract resolve_data_dir helper from get_db_path #729 ◀

_{Change managed by git-spice.}

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 55.66%. Comparing base (a5867f0) to head (d9f2e1d).
⚠️ Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/common/src/storage.rs	88.88%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #729   +/-   ##
=======================================
  Coverage   55.66%   55.66%           
=======================================
  Files         126      126           
  Lines       13759    13762    +3     
=======================================
+ Hits         7659     7661    +2     
- Misses       6100     6101    +1     

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[geoffjay]

👀 Conductor: Awaiting Reviewer

PR #729 (refactor(common): extract resolve_data_dir helper) has CI passing and is in the review-agent queue awaiting a reviewer pickup.

Nudge posted by conductor pipeline sync.

[geoffjay]

Review: refactor(common): extract resolve_data_dir helper from get_db_path

Stack position: issue-722 is directly on main with no parent PR
dependency, but git-spice reports (needs restack) — the branch is behind
main and must be rebased before the conductor can merge it.

The extraction of resolve_data_dir is the right refactor — sharing the
AGENTD_ENV-based path resolution between get_db_path and future callers
is clearly valuable. Two issues need fixing before this can merge.

---

Blocking — test race condition

The PR adds four separate #[test] functions that each mutate
std::env::var("AGENTD_ENV") via set_var / remove_var:

#[test]
fn test_resolve_data_dir_development() { ... }
#[test]
fn test_resolve_data_dir_dev_shorthand() { ... }
#[test]
fn test_resolve_data_dir_test() { ... }
#[test]
fn test_resolve_data_dir_production_contains_project_name() { ... }

These will race against each other and against the existing
test_get_db_path_respects_agentd_env when cargo test runs them in
parallel (the default). The result is non-deterministic test failures.

The comment directly above these tests says:

// All AGENTD_ENV variants are exercised in a single test to prevent
// data races between parallel test threads that share the process
// environment.

The implementation contradicts that comment. The existing
test_get_db_path_respects_agentd_env test in this file demonstrates the
correct pattern: exercise all AGENTD_ENV variants sequentially inside a
single #[test] function so only one test holds the env-var mutation at a
time.

Fix: collapse the four new tests into a single
test_resolve_data_dir_respects_agentd_env function that sequences all
variants, following the same structure as test_get_db_path_respects_agentd_env.

---

Non-blocking — doc comment displacement

The original get_db_path doc comment (lines 23–40 in the current file)
describes db_filename, mentions create_dir_all, and gives macOS/Linux
path examples. After this PR the extracted comment will be attached to
the private resolve_data_dir function, leaving the public get_db_path
with no doc comment at all.

Suggestion: keep a condensed doc comment on get_db_path describing its
public contract (what it returns, when it errors), and let resolve_data_dir
have a brief internal note. The full path examples are more useful on the
public function.

---

Action required before merge

1. Fix the four separate env-var tests → single sequential test (blocking)
2. Rebase onto main:

git-spice branch restack
git-spice branch submit