docs: update product messaging and positioning

[cybertec-adrian]

Summary

Updates product messaging and positioning across the documentation and SPEC.md
to reflect the planned open-core model (Apache 2.0 base version plus a planned
paid tier with additional features), replacing prior statements that asserted
there was no paid tier / no open-core / no feature gating.

Type

• Bug fix
• New feature
• Refactor (no behaviour change)
• Documentation
• Test infrastructure
• Packaging / release

Tests

• make check passes locally (vet + race tests)
• New tests added for the changed behaviour, or there's a clear reason none exist
• Integration tests pass (make test-integration) where touched

Compatibility

• No on-disk manifest schema changes (or: schema bumped + 24-month back-read preserved)
• No CLI / API contract changes (or: documented + bumped)
• No new external dependencies (or: justified in the description)

Checklist

• Maintainer-attribution authoring (Author: Hans-Jürgen Schönig <hs@cybertec.at>)
• No AI-attribution lines anywhere
• CHANGELOG.md updated under the unreleased section
• Comments explain WHY (not WHAT) where the code isn't self-evident