refactor(install): address audit findings (stdlib reuse, dead-code, comment hygiene)

[BANANASJIM]

Addresses a batch of LOW-severity audit findings in src/cli/install/.

Changes

• plan.zig: replace hand-rolled ensureDirAll/ensureDir with std.fs.cwd().makePath (behavior-identical, absolute paths work; ~20 LoC removed).
• plan.zig: replace copyFile body with std.fs.copyFileAbsolute — atomic temp+rename, no partial/truncated destination on mid-copy failure, preserves source mode.
• plan.zig: userInGroup supplementary-group buffer 64 -> 1024 so heavy LDAP/AD memberships no longer silently miss the input-group hint.
• plan.zig: remove duplicated InstallPlan doc sentence; strip PR-N forward-compat markers.
• udev.zig: fix service-sentinel ISO date month off-by-one (stdlib Month enum is 1-based; + 1 produced 2..13).
• services.zig: buildSystemctlSystemArgv / buildSystemctlUserArgv now free already-duped strings on allocation error instead of leaking them.
• mappings.zig: gate config backup on has_target so a pure force/interactive add (no matching entry) no longer writes a spurious config.toml.bak.*.
• phase.zig / tests.zig: remove PR-N markers from comments.
• tests.zig: add regression test writeBinding force pure-add does not create backup (asserts new entry added, old preserved, no backup file).

Skipped findings

• plan.zig groupGid /etc/group 4096-byte cap: fix requires threading an allocator through groupGid/hostHasInputGroup/userInGroup + call sites; disproportionate to a hint-only edge (LDAP /etc/group >4KB). Left as-is.
• udev.zig generateUdevRules dead-code wrappers: not truly dead — exercised by 10+ tests via the documented _internals_for_tests boundary; removing them expands test code and churns many call sites with no behavior benefit.
• probeAndRebindDrivers O(N*M) sysfs: explicitly flagged near-zero impact (install-time only, tiny N/M); not worth a structural rewrite.

Test plan

• Docker oracle ./scripts/padctl-docker test -> EXIT=0.
• New backup test proven falsifiable: temporarily reverting the has_target gate makes exactly this test fail at expect(!found_bak).
• zig fmt --check (0.15.2) clean on all changed files.

refs: codebase audit

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: b7af5022-d736-43bb-b6d6-c2388c52f303

📥 Commits

Reviewing files that changed from the base of the PR and between 8e1c4d6 and 0da4e51.

📒 Files selected for processing (6)

• src/cli/install/mappings.zig
• src/cli/install/phase.zig
• src/cli/install/plan.zig
• src/cli/install/services.zig
• src/cli/install/tests.zig
• src/cli/install/udev.zig

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/audit-cli-install

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