harden(auth/cookies): __Host- prefix on all three auth cookies (MED-14, BREAKING)

Audit 2026-05-10 — close MED-14 from the HANDOFF.md backend batch
(item 5). The session, CSRF, and OIDC pre-login cookies all carry
the __Host- prefix; browsers now reject any subdomain attempt to
overwrite them.

Cookie name changes (BREAKING — existing sessions invalidate):
  - certctl_session       → __Host-certctl_session
  - certctl_csrf          → __Host-certctl_csrf
  - certctl_oidc_pending  → __Host-certctl_oidc_pending

The __Host- prefix requires Path=/ + Secure + no Domain attribute.
Post-login session + CSRF cookies already met all three. The pre-login
cookie's Path widened from '/auth/oidc/' to '/' to satisfy the prefix;
the cookie lives 10 minutes and is only consumed by the callback
handler, so the wider path scope is harmless.

Files touched:
  - internal/auth/session/domain/types.go — constant rename + comment
  - internal/auth/session/domain/types_test.go — assertion update
  - internal/api/handler/auth_session_oidc.go — pre-login set + clear
    paths widened from /auth/oidc/ to /
  - web/src/api/client.ts — readCSRFCookie now compares against
    '__Host-certctl_csrf'
  - CHANGELOG.md — Unreleased > Security (BREAKING) entry
  - docs/migration/oidc-enable.md — operator-facing detail of the
    one-time re-authentication window + GUI customization guidance

Operator impact: ONE re-login prompt per active session at the deploy
that lands this change. Subsequent logins issue the __Host-prefixed
cookie automatically. Existing bookmarked deep links work without
modification (cookies are path-scoped, not URL-scoped).

Refs: cowork/auth-bundles-fixes-2026-05-10/HANDOFF.md item 5
      cowork/auth-bundles-audit-2026-05-10.md MED-14

CHANGELOG.md

@@ -1,5 +1,27 @@
 # Changelog
 
+## Unreleased
+
+### Security (BREAKING)
+
+- **`__Host-` cookie prefix on all three auth cookies (Audit 2026-05-10 MED-14).**
+  The session cookie, CSRF cookie, and OIDC pre-login cookie are renamed from
+  `certctl_session` / `certctl_csrf` / `certctl_oidc_pending` to
+  `__Host-certctl_session` / `__Host-certctl_csrf` / `__Host-certctl_oidc_pending`
+  to gain browser-enforced subdomain-takeover protection (a `__Host-*` cookie can
+  only be set with `Path=/` + `Secure` + no `Domain` attribute, and the browser
+  rejects subdomain attempts to overwrite it). **Active sessions invalidate on
+  the rolling deploy that lands this change** — operators must re-authenticate
+  once after upgrading. The GUI's CSRF cookie reader was updated in lockstep.
+  See `docs/migration/oidc-enable.md` for operator-facing detail.
+
+### Security
+
+- **Pre-login cookie Path widened from `/auth/oidc/` to `/` (Audit MED-14
+  follow-on).** Required to satisfy the `__Host-` prefix's `Path=/` rule. The
+  cookie lifetime is unchanged (10 minutes) and only the callback handler
+  consumes it; the wider path scope is harmless.
+
 ## v2.1.0 - Auth Bundles 1 + 2: RBAC primitive + OIDC SSO + sessions ⚠️
 
 > **SECURITY: AUDIT YOUR API KEYS.**