docs: add sdd/roadmap.md index of all SDDs

[kraftbj]

Summary

Adds sdd/roadmap.md — a single-table index of every Spec-Driven Development doc in this repo, with one-line purpose, status (planning / in-progress / shipped), and pointers to the relevant Linear epic and GitHub PR per row.

Also adds a one-line pointer in AGENTS.md so contributors land here from the natural starting point.

Why

The sdd/ directory has grown to 10 features on trunk plus 3 in-flight (PRs #90, #92, #93). Without an index, it's hard for a new contributor — or future-me — to know what exists, what's shipped, and what's in flight without reading 10 separate spec files. AGENTS.md already specifies that per-SDD plan.md carries a ## Progress checklist for task-level status; this roadmap rolls those up into a directory-level view.

Inputs

• Each directory under sdd/ on trunk (10 SDDs, drawn from their spec.md / requirements.md).
• The Linear "Radical Month: FOSSE" project — Linear refs were extracted per SDD from each spec rather than enumerated centrally.
• Open PRs against this repo whose branches touch sdd/: SDD: FOSSE-native posting UI #90 (FOSSE-native posting UI), SDD: Deactivation lifecycle #92 (Deactivation lifecycle), SDD: Bundled backends migration #93 (Bundled backends migration).

Test plan

• PHPCS / PHPUnit / ESLint / Prettier — N/A, docs-only diff
• Spot-check that every Linear issue link in the table resolves to an existing issue
• Confirm that adding a new SDD just means appending one row (the maintenance note at the bottom of roadmap.md should be enough guidance)