docs(licence): align prose SPDX headers to CC-BY-SA-4.0 (ShareAlike)

Owner ruling: prose documentation is CC-BY-SA-4.0 (ShareAlike),
confirming standards/LICENCE-POLICY.adoc Rule 1 / licensing-policy.toml
[default_docs]. Migrates 102 prose .adoc/.md files whose SPDX *header*
was the now-deprecated CC-BY-4.0 → CC-BY-SA-4.0. Code stays MPL-2.0.

Header line only — prose that merely *documents* an SPDX example (e.g.
docs/PMPL-NARRATIVE.adoc:77, wiki/Working-Rules.adoc) is untouched.
Local pre-commit hook updated to accept CC-BY-SA-4.0 (CC-BY-4.0
tolerated transitionally); owner-string diagnostic left strict.

Owner-authorised, per-file, single reviewable commit (not an
unprompted sweep). Operational/community-health files (CHANGELOG,
SECURITY, CONTRIBUTING, CODE_OF_CONDUCT, CLAUDE.md, DOCUMENTATION_STATUS,
_handoff) included uniformly; flip to MPL-2.0 on request.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: hyperpolymath