feat(scripts): add SPECIFY_NO_PERSIST env var to suppress feature.json writes (#4128)

[chelsealong]

Problem

Fixes #4128.

When multiple Spec Kit script invocations run concurrently against the same
checkout (e.g. multiple agents/subagents each working on a different
feature), each one typically sets its own SPECIFY_FEATURE_DIRECTORY. Most
core scripts (setup-plan, setup-tasks) call get_feature_paths() without
--no-persist, so every invocation persists its SPECIFY_FEATURE_DIRECTORY
to the shared .specify/feature.json — the last writer wins, silently
overwriting another process's pinned feature directory.

--no-persist (added for #3025) already exists as a per-call opt-out, but
it's a parameter each call site has to pass — scripts that don't know they
need it (like setup-plan/setup-tasks) never do.

Fix

Add SPECIFY_NO_PERSIST as the environment-level equivalent of
--no-persist / -NoPersist / no_persist=True. When set to 1 or true,
it suppresses the .specify/feature.json write in get_feature_paths()
regardless of whether the calling script passes the per-call flag. An
orchestrator can set it once for a whole process tree so no script
invocation in that tree can write feature.json, even ones that don't know
to opt out themselves.

Implemented identically in all three script variants per the project's
parity rule (AGENTS.md):

• scripts/bash/common.sh — get_feature_paths()
• scripts/powershell/common.ps1 — Get-FeaturePathsEnv
• scripts/python/common.py — get_feature_paths()

Also documented the new variable in docs/reference/core.md's environment
variables table, next to SPECIFY_FEATURE_DIRECTORY.

Fully backward compatible: existing behavior when SPECIFY_NO_PERSIST is
unset is unchanged in every scenario.

Test plan

Added tests/test_specify_no_persist.py, covering all three script
variants via setup-plan (which calls get_feature_paths() without
--no-persist, so it's a script that previously had no way to opt out):

• Baseline: SPECIFY_FEATURE_DIRECTORY set, no SPECIFY_NO_PERSIST →
  feature.json is written (existing behavior, unchanged).
• SPECIFY_NO_PERSIST=1/true → feature.json write is suppressed, for
  bash, PowerShell, and Python.
• Two-agent race: agent A persists specs/001-a, agent B runs with
  SPECIFY_FEATURE_DIRECTORY=specs/002-b and SPECIFY_NO_PERSIST=1 →
  feature.json still points at agent A's specs/001-a (the pin isn't
  clobbered).

Confirmed the new tests fail without the fix (git checkout HEAD~1 -- scripts/bash/common.sh scripts/powershell/common.ps1 scripts/python/common.py,
then re-ran):

tests/test_specify_no_persist.py::test_bash_persists_by_default PASSED
tests/test_specify_no_persist.py::test_bash_specify_no_persist_suppresses_write FAILED
tests/test_specify_no_persist.py::test_bash_specify_no_persist_does_not_clobber_existing_pin FAILED
tests/test_specify_no_persist.py::test_ps_specify_no_persist_suppresses_write FAILED
tests/test_specify_no_persist.py::test_py_specify_no_persist_suppresses_write FAILED
4 failed, 1 passed in 1.27s

And pass with the fix restored:

tests/test_specify_no_persist.py::test_bash_persists_by_default PASSED
tests/test_specify_no_persist.py::test_bash_specify_no_persist_suppresses_write PASSED
tests/test_specify_no_persist.py::test_bash_specify_no_persist_does_not_clobber_existing_pin PASSED
tests/test_specify_no_persist.py::test_ps_specify_no_persist_suppresses_write PASSED
tests/test_specify_no_persist.py::test_py_specify_no_persist_suppresses_write PASSED
5 passed in 1.28s

Also ran:

• shellcheck --severity=error on all tracked .sh files — clean.
• markdownlint-cli2 on docs/reference/core.md — same 2 pre-existing
  MD028 findings as on unmodified main (unrelated blockquotes further
  down the file, confirmed by diffing lint output before/after).
• Full pytest tests/ -q (6944 tests): 6921 passed, 9 skipped, 4 pre-existing
  failures unrelated to this change (test_presets.py,
  test_resolve_template_python_parity.py — template-composition/locale
  edge cases), confirmed present on main before this change too.

No manual agent-driven testing was done for this change since it's a
core-script/docs-only change with no slash-command template modifications.

AI disclosure

This PR was written primarily by an autonomous AI coding agent (Claude Code,
Claude Sonnet 5), including the code change, tests, and this description.