fix: tell review agents which project root to read files from - #363

Pre-existing code-quality issues surfaced by the reviewers on this PR
(none were introduced by the original fix, but pulling them in here
since the files are already under review):

instructions.py:
- _run_precompute_command docstring no longer claims "command" is an
  absolute path — it's a shell command string, invoked via shell=True
- Noted the trust model for precompute shell commands (repo-sourced,
  never fed untrusted input)
- ThreadPoolExecutor now caps at max_workers=8 so a repo with many
  distinct precompute commands can't briefly fork dozens of shells
- _build_reference_files_section no longer re-encodes the (possibly
  truncated) content string to recompute its byte count; the consumed
  budget is now tracked explicitly as either original_byte_len or the
  exact remaining budget on truncation

config.py:
- ReviewRule.precomputed_info_bash_command field comment matches the
  docstring fix above

test_instructions.py:
- Moved five traceability comment blocks from inside the method body
  to the two lines directly preceding the def line, matching the
  convention enforced by test_file_quality
- test_count_cap_triggers_omission no longer hardcodes "f20.txt" as
  "one of the omitted" — derives the filename from MAX_INLINE_FILES
  so the test stays correct if the constant changes
- _sanitize_for_id import moved from a function-local line to the
  top-of-module import block

REVIEW-REQ-005 spec:
- Split five compound requirements so each numbered item carries
  exactly one RFC 2119 keyword (requirements_file DeepSchema rule).
  Preserved existing IDs (5.1.4, 5.1.8, 5.8.4–5.8.6) — each retains
  its primary MUST — and appended the secondary obligations as new
  sequential IDs (5.1.10, 5.1.11, 5.8.7–5.8.11)

CHANGELOG.md:
- Added entry for the worktree file-path fix under the Unreleased
  Fixed section

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

instructions.py:
- precomputed_info lookup now uses direct key access ([]) instead of
  .get() so a dict/filter drift between unique_commands and the per-task
  lookup would fail loudly instead of silently dropping precomputed
  context. Also aligned both predicates on `is not None` so an
  empty-string command would not be executed then silently discarded.
- Added a docstring note to _content_hash pinning the trust contract:
  files entries must be repo-root-relative paths from trusted
  .deepreview config, with no absolute paths or .. traversal.

test_instructions.py:
- New module-level `instructions_dir` pytest fixture that creates
  `.deepwork/tmp/review_instructions/` under tmp_path. Seven tests now
  use it instead of repeating the two-line setup inline.
- test_single_file_produces_expected_format uses direct
  `(tmp_path / "src").mkdir(...)` instead of the indirect `.parent.mkdir`
  form, matching adjacent tests in TestComputeReviewId.
- Added an inline comment on the `2 * MAX_INLINE_FILES` fence-count
  assertion explaining the 2x relationship.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>