[109c] start resolution chain (project → user → prompt-init) + --scope

[fedorovvvv]

Sub-issue of #109.

Goal

npx @forgeplan/web start should Just Work — find a scaffold automatically, prompt to init if none exists.

Scope

• Resolution order: <cwd>/.forgeplan-web/ → ~/.forgeplan-web/ → prompt user to run init.
• --scope user|project overrides resolution (force the chosen scope, fail loudly if absent).
• Honor FORGEPLAN_CWD env (current behavior preserved).
• New helper: resolveScaffold({ cwd, scope }) → { path, scope, configPath } | null.

Forgeplan

• PRD-026 + RFC-021

Acceptance

• cd /any/dir && npx @forgeplan/web start starts user-scope server when project-scope absent.
• --scope project in a dir without .forgeplan-web/ errors clearly without falling back to user.
• No .forgeplan-web/ anywhere → prompt offers init --scope user|project interactively.
• Existing project-scope users see no behavior change.

Depends on

• 109b (scope flag, user-scope path resolver)

[fedorovvvv]

Merged via #118 (commit a6fcaef).