[fix] stack redeploy rebuilds an edited build context

[malickyeu]

Summary

StackRedeploy ran a plain docker compose up -d, so a CLI-discovered stack that
declares build: kept running the image from its first deploy however much its
Dockerfile or build context changed on the host — while the CLI reported
Container Running and exited 0.

This is the same staleness bug fixed for Projects in #99, on the other code
path. A project deploys through the API handler; a CLI stack deploys through
StackRedeploy. Fixing one didn't fix the other, and nothing pointed that out —
it surfaced while auditing the docs against the merged code, because the doc I was
about to write said up -d and I checked why.

Redeploy now passes --build, which is a no-op for services that only pull an
image.

Stacked on #104 (docs/post-merge-truthing), which introduced the NEXT.md
entry this PR removes. Merge #104 first, or retarget this to main and expect a
one-line NEXT.md conflict.

Type of change

• Bug fix
• New feature
• Docs only
• Refactor / chore

Checklist

• go test -short ./... and go vet ./... pass
• gofmt gate is clean (gofmt -l $(git ls-files '*.go') after staging)
• Frontend type-checks (cd web && npx tsc --noEmit)
• Rebuilt and committed web/dist
• Added/updated tests for the change
• Updated docs/ and added a CHANGELOG.md entry

Notes for reviewers

The test creates the stack with the real docker compose CLI, not with
hand-written labels. That matters here: labels are the only thing
StackRedeploy has to work from — project name, config_files, working_dir —
so a fixture that writes them by hand would be testing my idea of what compose
stamps rather than what it actually stamps. It then edits the context on disk and
redeploys through the app.

Mutation-verified, per the discipline recorded in #104: dropping --build
fails the test with marker is "v1", expected "v2". It fails for the right
reason — the image genuinely wasn't rebuilt — rather than on an unrelated error.

Also updated the UI's confirm dialog, which promised docker compose up -d.
It now says up -d --build and mentions that build: services are rebuilt, so
the dialog still describes what the button does.

Not given an opt-out, unlike the project deploy's {"build": false}: the
stack redeploy endpoint takes no body, and inventing one for a case that costs
nothing on image-only stacks seemed worse than the consistency. Easy to add if a
large-context stack over a slow link ever makes it worth it.

Test data: the stack (dctest-stackredeploy) and its built image are removed
in t.Cleanup, and the whole project label is swept with freeStack before and
after — the pattern added in #100 after a failed guard left containers behind.
Verified zero dctest* leftovers across the run.

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