Add doc/requirements/, a structured set of normative requirements
extracted from the current ocserv implementation (internal/*.md,
generated with the requirements-from-implementation protocol) and
from the OpenConnect/AnyConnect protocol sources, reconciled into
protocol/unified.md.
Update AGENTS.md so that new features and bug fixes are documented as
requirements first: find or add the relevant REQ-* entry (with
acceptance criteria) and update the implied tests before changing
code, and confirm in merge requests that existing requirements and
use-cases still hold.
This follows partially https://github.com/microsoft/PromptKit
Signed-off-by: Nikos Mavrogiannopoulos <n.mavrogiannopoulos@gmail.com>