Document the forum port: user guide and architecture notes #30

Outcome

The docs describe the new Phoenix forum surface: what it is, where legacy content went, how identity linking works, and how the port maps to the old Effect implementation.

Work

• User-facing page under priv/docs/ (and wherever the docs catalog lists it): reading, posting, claiming a legacy identity.
• Architecture note covering schema mapping, state normalization (sticky→pinned, tombstoned→deleted), skipped pipeline posts, and the cutover path; reference docs/evidence/forum-port-migration.md.
• Update docs/taxonomy.md if any new terms need defining.
• Add the forum pages to the docs catalog so they are discoverable.

Acceptance criteria

• A person who only knew the old forum can find their topics and claim their identity by following the docs alone.

Verification

Docs catalog tests; review against the Google style guide.