feat(verbs): new built-in exec verbs & custom verb management

[jahvon]

Summary

Verbs have been a fixed list. Users can now register their own verbs in their user config. The built-in list
also grows to cover missing counterparts and common AI/ML workflows.

Notable Changes

• flow config add verb NAME... / flow config remove verb NAME... manage a new customVerbs
  config field. Names that clash with a built-in verb or a flow command are rejected.
• New built-in verbs: resume, unlock, pull, upload, download, diff, plan, render,
  train, tune, evaluate, infer, rev is an alias of evaluate`.

Closes #473

[codecov]

Codecov Report

❌ Patch coverage is 84.43114% with 26 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cmd/internal/config.go	79.26%	12 Missing and 5 partials ⚠️
internal/validation/validate.go	85.71%	2 Missing and 2 partials ⚠️
cmd/internal/exec.go	77.77%	1 Missing and 1 partial ⚠️
types/config/config.go	71.42%	2 Missing ⚠️
main.go	0.00%	1 Missing ⚠️

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