A registry of labs: --lab on every command, the one lab as the default, a picker among several (an agentlab.yaml in the current directory still wins)

[teemow]

Part of #119.

Problem

A lab is a directory — agentlab.yaml, certs/, state/ — and every command reads it from the current directory: config.Load reads config.File relative to it, caCertPath/tlsCertPath (certs.go), labKubeconfig() (kubeconfig.go) and some thirty other sites are cwd-relative. The binary does not know which labs exist on the machine, so list (#121) has nothing to list, and open, pods or logs from anywhere else fail with "no agentlab.yaml found". With one lab — the common case — the person still has to cd into it first.

Proposed solution

• A registry under os.UserConfigDir()/agentlab/ (~/.config/agentlab/ on Linux, ~/Library/Application Support/agentlab/ on macOS; the sibling of the update check's cache directory): one entry per cluster name → the absolute lab directory. configure and up write or refresh the entry for their lab; reading prunes entries whose directory or agentlab.yaml is gone. down leaves the entry — the lab still exists as a directory, and list shows its cluster as not created.
• One resolution order, in one place (loadConfig in main.go, so every command gets it): --lab <name> (a persistent flag on the root) → an agentlab.yaml in the current directory → the single registered lab → on a terminal a huh select among the registered labs (the form's accessible mode applies: ACCESSIBLE, --accessible) → off a terminal a refusal that names the labs and --lab. A --lab that is not registered is refused naming the registered ones. Commands that touch no lab (self-update, completion, help, and list itself) never resolve one.
• The command then runs against that lab's directory. The smallest change that keeps the cwd-relative sites correct is to change into the lab directory before anything touches a path; a lab-directory root threaded through the package is the cleaner shape and can follow. Either way nothing moves and nothing is written outside the lab directory and the registry.
• A name collision is refused. configure in a directory whose clusterName is registered to a different directory stops, naming both directories and asking for another clusterName — a second lab of that name would collide on the cluster and its ports anyway (today the discovery then treats the other lab's published ports as its own).
• agentlab.yaml's clusterName is the lab's name everywhere: the registry, --lab, list, the picker.

Acceptance criteria

• With one registered lab, agentlab pods and agentlab logs muster from an unrelated directory use it. With two, a terminal gets a picker and < /dev/null gets a refusal naming both labs and --lab; --lab <name> skips the picker.
• In a lab directory the local agentlab.yaml is used without a question, whatever the registry says.
• configure with a clusterName registered to another directory is refused naming both directories.
• Entries whose directory has vanished are neither offered nor listed.
• Unit tests cover the resolution order, the pruning and the refusal; the picker is driven like the form's tests (internal/forms, scripted keystrokes).
• docs/cli.md's intro paragraph (today: "every command reads agentlab.yaml…") describes the order above and --lab.