docs: add ROADMAP with a free-stays-free commitment

[parth0025]

S0-05 — Free/paid boundary + public roadmap (+ CLA steps)

Restores ROADMAP.md (it was removed earlier, leaving broken README links) with the Sprint-0 "irreversibility insurance":

• Free stays free commitment — everything that works today (all views incl. upcoming Gantt, time tracking + screenshots, custom fields, custom roles, dashboards, AI with your key, webhooks, importers, sprints, milestones, chat) stays free under AGPL-3.0.
• What may become paid — only new enterprise add-ons (SAML/SCIM/LDAP, audit-log API, managed cloud, SLA support), never a take-back of free features.
• Now / Next / Later direction section + how to suggest features.

This also makes the README roadmap references resolve again (PR #241 temporarily pointed them at Discussions; once both merge they can point back to ROADMAP.md).

Remaining S0-05 steps that are external (not repo PRs) — for you to action

1. Pin the "free stays free" statement as a GitHub Discussion (Announcements).
2. CLA — this is a legal decision I deliberately did not commit:
   • Recommended: install the CLA Assistant GitHub App and paste a standard Individual CLA granting Aliansoftware relicensing rights (needed if an enterprise edition is on the table).
   • Lighter alternative: enable DCO sign-offs via the dco GitHub App.
   • Tell me which and I can add the matching workflow/CLA.md in a follow-up PR.
3. Public Project board — convert the sprint backlog into a public GitHub Project.

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