Motivation

lstk is increasingly invoked by AI agents and scripts, which can produce malformed or hostile input (control characters, path traversal, percent-encoding, shell metacharacters) in ways humans rarely do. Pod names, the auth token, and [env.*] config values were only loosely checked (or not at all), so bad input surfaced as a confusing error deep inside a Docker call or platform API request instead of failing fast with a clear reason.

Changes

• Add internal/validate, reusable deterministic validators (ResourceName, EnvVarName, NoControlChars, AuthToken) that return a machine-classifiable *Error (stable Rule field) instead of a generic message.
• Wire validate.AuthToken into cmd/root.go's token resolution (env/keyring): trims whitespace, then rejects control chars, embedded whitespace, or implausibly long tokens.
• Wire validate.EnvVarName/NoControlChars into internal/config/config.go to reject malformed [env.*] keys/values before they're injected into a container's environment.
• Replace internal/snapshot's bare pod-name regex with validate.ResourceName, adding deny-checks for percent-encoding, path traversal, embedded path/query characters, and shell metacharacters.
• Rebase conflict resolution: current main had independently grown its own pod-name validator (ValidatePodName, from the S3 remote-storage PR Add S3 remote storage for snapshot save, load, and list #344) that permits underscores and is also called directly for S3-remote pod names in cmd/snapshot.go. ValidatePodName now delegates to validate.ResourceName, whose charset was widened to keep underscores working — the S3-remote pod-name path now gets the same deny-checks as pod: refs, which it didn't before.
• Document the new package in the root CLAUDE.md, and encode its usage as a convention in the agent instructions so future input surfaces route through it instead of growing parallel validators: a Code Style rule in CLAUDE.md (route by value class — identifiers → ResourceName; opaque secrets → loose AuthToken-style checks; paths/URLs → their existing parsers; extend internal/validate rather than inlining a regexp), an input question + wiring/test steps in the add-command skill, and a checklist item in the review-pr skill.

Review

Human review advised — changes user-facing error behavior for auth tokens, pod names, and [env.*] config (previously-accepted values are now rejected with new/different messages), and this rebase merges two independently-evolved pod-name validators.

Tests

New unit tests in internal/validate/validate_test.go (all validators, deny-check ordering, rule codes) and internal/config/env_validation_test.go; extended internal/snapshot/destination_test.go with malformed pod-name cases (percent-encoding, embedded query, shell metacharacters) plus a regression case confirming underscore-named pods still pass. Verified after rebase: make build, go vet ./..., make test (full suite), make lint (both modules) all green.

Closes PRO-306