Global `-C <path>` / `--repo` and a single `Workspace` path resolver

[Max-Levitskiy]

Priority: P0 · Epic: #71 · Refs: R27

Why

Every command hardcodes Path::new("weaver.yaml"), weaver.lock, .rw/state.yaml,
.rw/answers.yaml relative to cwd (apply.rs:54,70,113,334, check.rs:15,
describe.rs:24, list.rs:24, run.rs:28, module.rs:65,107,140,165,
plugins.rs:46,90,145,213). CI, fleet runs, and foreign-repo validation all need to
point wvr at another directory.

What

• weaver_core::workspace::Workspace { root, config_path, lock_path, state_path, answers_path, plan_dir } with Workspace::discover(root: Option<PathBuf>).
• Global clap args -C/--repo <path> (also WEAVER_ROOT); relative --plan/--out
  paths resolve against the invocation cwd, everything else against root.
• All commands take a &Workspace; PluginResolver::new(root) and
  ModuleResolver receive it instead of PathBuf::from(".").

Acceptance

• Test: wvr -C /tmp/x plan on a workspace in /tmp/x from another cwd behaves
  identically to running inside it, for plan, apply, check, describe, list,
  run, module list.
• grep -rn 'Path::new("weaver.yaml")' crates/cli returns nothing.