chore(spdx): add Copyright (c) headers to Markdown files (batch 1/7) (#95)

Mechanical sweep: inject SPDX-FileCopyrightText / Copyright (c) lines
into 105 .md files that lacked them. Comment block uses HTML
`<!-- ... -->` style at file top.

Stats:
- 106 .md files in scope
- 1 already had Copyright (PROOF-NEEDS.md, from PR #47)
- 8 had SPDX but missing Copyright (line inserted after SPDX)
- 97 had neither (4-line block prepended)
- 2 files removed from main pre-sweep (CHANGELOG.md, MAINTAINERS.md)

This is batch 1/7 of the SPDX-FileCopyrightText sweep originally
queued in the M1 foreign-WIP triage report.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath