refactor: optional DI pattern + documentation

[KSXGitHub]

Summary

Pick a coherent set of identifiers for the dependency-injection seam introduced in PRs #332, #333, and #337, apply them across every affected crate, and write the pattern down in CODE_STYLE_GUIDE.md so future PRs have a single reference point.

Naming:

• Api (generic parameter) → Sys. The provider is a system seam, not an API surface.
• RealApi (production type) → Host. Drops the Platonic Real prefix; production stays unqualified, fakes carry the qualifier.
• FsReadString (cmd-shim) → FsReadToString, unifying with the trait already shipped in modules-yaml and matching std::fs::read_to_string.

Documentation:

• New ### Dependency injection for tests section in CODE_STYLE_GUIDE.md (near ### Unit test file layout) — the eight principles, a worked filesystem example anchored to crates/modules-yaml, cross-domain composition under one generic, and when a real fixture is simpler than a fake.
• AGENTS.md gains a pointer to the new section.

Crates touched: modules-yaml, cmd-shim, config, reporter, package-manager, cli.

Consolidation evaluation: Deferred to a follow-up. The four Host declarations are similar enough to fold into a shared crates/api, but the move involves a new crate plus redistributing the walkdir / httpdate / gethostname dependency footprint — a cleaner diff on its own. Renaming first is the prerequisite that follow-up needs.

Test plan

• cargo fmt
• cargo check --locked
• cargo clippy --locked -- --deny warnings
• Test suites for modules-yaml, cmd-shim, config, reporter pass locally.

Resolves #339.

---

Written by an agent (Claude Code, claude-opus-4-7).

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.51%. Comparing base (fe27251) to head (e77ea79).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #517   +/-   ##
=======================================
  Coverage   88.51%   88.51%           
=======================================
  Files         125      125           
  Lines       13821    13821           
=======================================
  Hits        12234    12234           
  Misses       1587     1587           

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[github-actions]

Micro-Benchmark Results

Linux

group                          main                                   pr
-----                          ----                                   --
tarball/download_dependency    1.00      8.1±0.40ms   532.5 KB/sec    1.01      8.3±0.26ms   525.4 KB/sec

[github-actions]

Integrated-Benchmark Report (Linux)

Scenario: Frozen Lockfile

Command	Mean [s]	Min [s]	Max [s]	Relative
pacquet@HEAD	2.673 ± 0.095	2.553	2.824	1.00 ± 0.05
pacquet@main	2.668 ± 0.074	2.555	2.757	1.00
pnpm	6.426 ± 0.145	6.199	6.762	2.41 ± 0.09

BENCHMARK_REPORT.json

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Scenario: Frozen Lockfile (Hot Cache)

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
pacquet@HEAD	748.7 ± 31.5	717.2	825.2	1.00
pacquet@main	856.6 ± 67.1	788.9	978.9	1.14 ± 0.10
pnpm	2653.5 ± 66.8	2588.7	2779.5	3.54 ± 0.17

BENCHMARK_REPORT.json

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