Add `provisionhook.EnvMutator` extension point so out-of-tree plugins
(e.g. github-app-auth, vault-secrets) can inject or override env vars
right before container Start, without forking core or piling more
provider-specific code into the handlers package.
WorkspaceHandler gains an optional `envMutators *provisionhook.Registry`
wired in via SetEnvMutators during boot. The hook fires after built-in
secret loads + per-agent git identity, so plugins can both read what's
already there and override anything they own (GIT_AUTHOR_*, GITHUB_TOKEN).
A nil registry is a no-op via Registry.Run's nil-receiver branch — keeps
the hot path a single nil compare and means existing flows stay green
even with zero plugins registered.
Mutator failure aborts provisioning and marks the workspace failed with
the wrapped error in last_sample_error. Failing fast surfaces the cause
to the operator instead of letting an agent boot into opaque "git push
401" loops it can never recover from on its own.
Tests cover ordered execution, chained env visibility, first-error abort,
nil-receiver no-op, nil-mutator drop, registration order, and concurrent
register-vs-run safety (-race clean).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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