refactor(cli): consolidate duplicated path-strip + output-routing onto shared helpers

[dekobon]

Summary

Cleanup surfaced while reviewing #544. Now that
big-code-analysis-cli/src/format_util.rs::strip_path_prefix and
lib.rs::{validate_output_path, write_output_or_stdout} exist as shared
helpers, three pre-existing inline duplicates should migrate onto them. This is
a pure DRY refactor — identical logic, no behavior change.

Duplication to consolidate

1. Path-prefix stripping (→ format_util::strip_path_prefix):

   • big-code-analysis-cli/src/exemptions.rs — private fn strip (~line 335)
     and its 6 call sites (lines ~139, 163, 194, 214, 228, 275).
   • big-code-analysis-cli/src/markdown_report.rs — inline
     file.strip_prefix(strip_prefix).unwrap_or(file) (~line 123).
     All three are byte-identical to strip_path_prefix.

2. Output-path validate + write (→ validate_output_path /
   write_output_or_stdout):

   • commands.rs::run_command_report (validate block ~1585, write ~1614).
   • commands.rs::run_command_exemptions (validate ~2158, write ~2194).
     The new validate_output_path emits the same error wording
     (--output must be a file path for \`, parent directory of
     --output does not exist: …); confirm the write-path die_io` verb matches
     the existing message before swapping (keep the message identical).

Acceptance

• exemptions.rs and markdown_report.rs call strip_path_prefix; the private
  strip fn is removed.
• run_command_report/run_command_exemptions use the shared validate/write
  helpers with byte-identical error messages.
• No behavior change; existing report/exemptions tests pass unmodified.

Found during #544 review; additive/non-breaking, no milestone.

---

Resolution

Fixed in e28cd710 (branch fix/issue-545). Pure DRY refactor, no
behavior change.

• exemptions.rs: the 6 strip(...) call sites now call
  format_util::strip_path_prefix; the private strip fn is removed.
• markdown_report.rs: the inline
  file.strip_prefix(strip_prefix).unwrap_or(file) in
  extract_summaries now calls strip_path_prefix. The shared helper's
  empty-prefix guard yields an identical result for the default empty
  prefix.
• commands.rs: run_command_report and run_command_exemptions now
  use validate_output_path and write_output_or_stdout. Error wording
  (--output must be a file path for \`, parent directory of
  --output does not exist: …) and write verbs (write report to, write exemptions report to`) are byte-identical to the prior inline
  blocks.

Validation: cargo fmt --all -- --check, cargo clippy --workspace --all-targets -- -D warnings, and cargo test --workspace --all-features
all pass. Existing report/exemptions tests pass unmodified; no .snap.new
drift.

[dekobon]

Resolved on branch fix/issue-545 in commit e28cd710.

The three inline duplicates now call the shared helpers:

• exemptions.rs (6 call sites) and markdown_report.rs use
  format_util::strip_path_prefix; the private strip fn is deleted.
• run_command_report / run_command_exemptions use
  validate_output_path + write_output_or_stdout with byte-identical
  error messages and write verbs.

Pure DRY refactor, no behavior change. fmt, clippy --workspace --all-targets -D warnings, and test --workspace --all-features all green; existing report/exemptions tests pass
unmodified. Leaving open for the orchestrator to close after merge.