Cut a pre-0.1.0 tag and write a downstream migration guide (stopgap before crates.io)

[macanderson]

Context

Until #16 (publish to crates.io) ships, downstreams pin git revs — and today they pin badly: stella pins rev 7912257c against the PRE-RENAME repo URL (works only via GitHub's rename redirect; tracked as stella#277), and oxagen vendors an un-renamed copy (oxagen-platform#1082). There is no tag to pin, no changelog to migrate against.

Task

• Tag the current trunk (e.g. v0.0.x) so git pins can be by-tag rather than by-sha.
• MIGRATION.md: old URL → new URL, ocp-* → contextgraph-* symbol map, and the breaking changes between 7912257c and the tag.
• Note the redirect hazard explicitly (a future repo named opencontextprotocol would silently capture old pins).

Acceptance

• Both downstream issues can be executed using only the tag + MIGRATION.md, no code archaeology.