feat(doubles): let bashunit::mock accept an exit code like bashunit::spy

[Chemaclass]

Context

bashunit::spy accepts an optional second argument that is either an exit code or
a replacement implementation (src/test_doubles.sh:66-96):

bashunit::spy curl 1        # record calls AND return 1

bashunit::mock has no equivalent. Making a mocked command fail requires a
throwaway helper function:

function _fail() { return 1; }
bashunit::mock curl _fail

or an inline body whose exit code is easy to get wrong, because
bashunit::mock cmd echo something exits with echo's status (0), not the
status the caller wanted to simulate.

Proposal

Teach bashunit::mock the same all-digits second-argument convention spy
already uses, so the two doubles are symmetric:

bashunit::mock curl 1                  # exits 1, no output

Reuse the exact case glob from bashunit::spy ('' | *[!0-9]*) — it is what
keeps the interpolated value provably numeric and so keeps return $code free of
shell injection. Do not reimplement the check.

Open question worth settling in the issue discussion: how to express "output X
and exit 1" in one call. Options: a third argument, or leave it to the
existing replacement-implementation form. Prefer not to grow the signature unless
there is demand.

Why this matters for agentic coding

Error-path tests are the ones an agent is most often asked to add ("what happens
when curl fails?"). The current answer requires a helper function whose only job
is return 1, which agents both write inconsistently and shadow across tests.
A documented one-liner removes a step and makes the two doubles learnable as one
rule instead of two.

Acceptance criteria

• bashunit::mock cmd 1 makes cmd return 1
• bashunit::mock cmd 0 returns 0
• Existing forms unchanged: bashunit::mock cmd echo hi,
  bashunit::mock cmd <<< "out", bashunit::mock cmd some_function
• A command whose name is all digits is not silently reinterpreted (guard
  the argument position, not the command)
• Tests in tests/functional/doubles_test.sh
• Documented in docs/test-doubles.md + docs/public/bashunit-skill.md
• Bash 3.0+ compatible, make sa + make lint green

[Chemaclass]

Done in #907 (merged).