CI: Default SESSIONS_WEB_SMOKE_GATE=1 in ci-fast after #1887 green

[PurHur]

Problem

#1887 defines VM web-smoke + ExamplesCompileTest coverage for 005-SessionsWeb behind SESSIONS_WEB_SMOKE_GATE=0 today. Without a follow-up gate issue, the ladder stays opt-in forever (same pattern as #1888 for nested return / #1869 for MiniWebApp JIT).

Goal

When #1887 curls are green on harness master:

1. Flip SESSIONS_WEB_SMOKE_GATE="${SESSIONS_WEB_SMOKE_GATE:-1}" in script/ci-defaults.env
2. Wire into script/ci-fast.sh after existing examples-web-smoke (DevEx: script/examples-web-smoke.sh — curl harness for shipped web examples #298)
3. List gate in docs/local-ci-matrix.md and phpc doctor --gates (DevEx: phpc doctor --gates lists north-star1-verify presenter steps (#1845 phase 2) #1857 pattern)

Acceptance criteria

./script/ci-fast.sh   # runs 005 session curls by default
SESSIONS_WEB_SMOKE_GATE=0 ./script/ci-fast.sh   # opt-out for iteration

Verification (local / Docker only)

docker run --rm -v "$(pwd):/compiler" -w /compiler php-compiler:22.04-dev ./script/ci-fast.sh

Dependencies

• Testing: 005-SessionsWeb in examples-web-smoke + ExamplesCompileTest #1887 — smoke script + PHPUnit discovery
• Examples: 005-SessionsWeb — session_start + flash message reference app #1881 — example routes

Links

• CI: Default NESTED_RETURN_COMPLIANCE_GATE=1 in ci-fast after #1885 #1888 — nested return gate pattern
• CI: Default MINIWEBAPP_JIT_PROJECT_GATE=1 in ci-local after #587 green #1869 — MiniWebApp JIT gate pattern
• ROADMAP #78

[PurHur]

Done in PR #1917: script/ci-defaults.env sets SESSIONS_WEB_SMOKE_GATE default 1; ci-fast.sh / ci-local.sh run examples-web-smoke.sh --sessions-only via ci_run_sessions_web_smoke. Opt-out: SESSIONS_WEB_SMOKE_GATE=0.