docs: add quickstart and troubleshooting docs

[stuft2]

Summary

• add a new Quickstart (1 minute) section near the top of the README with a copy/paste local success path and optional Vault example
• add a Troubleshooting section using symptom -> cause -> fix entries mapped to real runtime/CLI error strings
• document precedence behavior with an explicit example (existing env wins)
• add a verbose-mode safety note and link the feature in the roadmap
• update Taskfile quality-gate commands for golangci-lint v2 and sandbox-safe caches

Test evidence

• task fmt (pass)
• task lint (pass, 0 issues)
• task test (pass)

Risk / rollback

• Risk is low: changes are docs + task automation config only (no runtime code paths changed).
• Rollback: revert commit 903b602 or close this PR without merging.