fix(orchestrator): generic 'build was cancelled' error hides actual failure cause

Problem

When a template build is cancelled or times out, the orchestrator returns only a generic build was cancelled message. This hides the actual failure cause (e.g., envd crash, provisioning failure) and makes debugging very difficult for users.

Root Cause

In builderrors.WrapContextAsUserError(), any context.Canceled or context.DeadlineExceeded error is replaced with a bare ErrCanceled / ErrTimedOut, discarding the entire error chain that contains diagnostic information.

Example

When envd crashes during a build, the actual error chain is:

wait for envd: failed to init new envd: failed to init envd: context canceled

But the user only sees:

build was cancelled

Proposed Fix

Three targeted changes (+53/-9 lines):

1. PhaseBuildError.Error() — include phase/step context in the error message
2. phases.Run() — wrap context cancellation errors with the active phase metadata
3. WrapContextAsUserError() — preserve the original error chain instead of replacing it

Before / After

Fully backward compatible — errors.Is(err, ErrCanceled) still works.

Start by reading builderrors.WrapContextAsUserError(), PhaseBuildError.Error(), and phases.Run(), which are the entry points named in the issue. Trace how cancellation and timeout errors move through these functions, then verify that phase and step context plus the underlying error chain appear in messages while errors.Is(err, ErrCanceled) remains compatible.