volt: verify gains a seventh leg -- doc-commands, graded at WORKTREE

The gate had six legs and none of them read a docstring's own command line.
doc_command_check.py existed since 838750c but only as a tool: nothing on the
gate ran it, so a documented flag its parser never learned could land and
nobody read it. This is that leg.

  doc-commands   tools/doc_command_check.py --rev WORKTREE --fail
                 WORKTREE, not HEAD: the commit window is where a docstring is
                 edited. Measured at the worktree this lands in: 67 commands
                 over 19 oracle tools, 67 OK, rc 0.

doc_command_check.py learns --rev WORKTREE: tracked paths from `git ls-files`
(an untracked tools/x.py is not part of the commit this mode grades), bytes
from disk. Outside a work tree it is rc 2, not an empty clean run. Selftest
15 -> 18 cells: one document edited on disk but not committed is rc 0 at HEAD
and rc 1 at WORKTREE, and the flag it names is the edited one.

Three tools stopped defaulting to a fixed path on this host. verify.py,
selftest_sweep.py, write_probe.py, citation_resolve_check.py,
path_claim_check.py and doc_census_check.py now resolve DEFAULT_REPO from the
file's own location, so a bare run in a clone grades the clone.

Three selftests refuse unreadable input at rc 2 instead of reporting it as
failed content: tool_map (not a work tree), write_probe (no table), and
citation_resolve_check (no corpus). Each cell below them graded `repo`, so an
absent tree read as "your table is wrong".

Evidence, all measured on this tree before the commit:
  verify.py --selftest              55 cells, 55 passed   (docstring says 55)
  doc_command_check.py --selftest   18 cells, 18 passed   (baseline pinned 18)
  tool_map.py --repo . --rev WORKTREE --check docs/TOOLS.md   rc 0
  verify.py --repo .                7 legs, 6 pass, 1 fault -- selftest-sweep,
                                    tool_map's drift cell, which renders HEAD
                                    and so cannot pass inside a commit window
tools/selftest_baseline.tsv re-pins doc_command_check.py 15 -> 18 cells.

tools/selftest_baseline.tsv

@@ -17,7 +17,7 @@ checkin_check.py	36	cells
 citation_hash_class.py	19	fixtures
 citation_resolve_check.py	14	fixtures
 doc_census_check.py	104	cells
-doc_command_check.py	15	cells
+doc_command_check.py	18	cells
 gate_blame.py	21	cells
 landing_check.py	26	cells
 path_claim_check.py	27	cells