#388 build: name the source-archive ceilings in the context check (SUP2-1094)

Fixes SUP2-1094. insta build --json said needs-attention with only a "large context" warning at 315.3 MB. The 256 MiB compressed upload cap first showed up at insta deploy, after packing (307.7 MiB > 256.0 MiB).

What changes

The context check in insta build now measures the context against the archive lane's limits (pack.ts ARCHIVE_LIMITS). Build stays offline, so these are labelled as defaults; the server's negotiated figures still win at deploy.

Case	Message
over the extracted-size or entry limit,	too large for insta deploy <dir>: …
already-compressed files (.gz/.xz/.zip/images/video…) alone, or payload plus tar headers/padding, over the upload limit	ships only if it compresses below the cap (extension-based estimate, never a certain verdict)
uncompressed size over the upload limit	X MiB uncompressed (Y of it already compressed) — ships only if it compresses below the cap
under the cap	unchanged

Uncompressed bytes alone never make the call, because a large text tree can pack small. When a ceiling applies, the next action also covers intentional large assets: recompress with xz/zstd, or fetch the file at start-up. That's the reporter's case: a checkpoint they ship on purpose.

--json gains archiveLimits: {maxArchiveBytes, maxExtractedBytes, maxFiles, source: "default"}.

Verification

• npm run typecheck passes. test/build.test.ts passes 41/41, including new cases: 315 MB reported case, compressible vs incompressible, extracted/entry limits, custom limits, compressed-file accounting.
• Ran it on real directories (300 MiB):
  • random checkpoint.gz → too large for insta deploy <dir>: already-compressed files alone are 300.0 MiB, over the 256.0 MiB compressed upload limit
  • zero-filled data.bin → ships only if it compresses below that
• npm test: test/setup-agent.test.ts fails 12 tests with ApiError: unauthorized. It fails the same way on unmodified main on my machine (local login state), so this PR doesn't cause it.

Docs: skills insta/cli-reference.md updated in a companion PR.

🤖 Generated with Claude Code

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Summary by cubic

The insta build context check now flags contexts that exceed the deploy archive ceilings at build time, instead of surfacing only at insta deploy after packing.

• Ceilings are measured against what the packer would actually archive (root .dockerignore, else .gitignore fallback), not the docker context, and count tar headers and gzip framing along with the payload.
• Over the extracted-size or entry limit → certain failure: too large for insta deploy <dir>. Uncompressed size alone never triggers a certain verdict, since compressible trees can pack well below the cap.
• Already-compressed files (.gz, .xz, .zip, images, video, etc.) alone over the upload cap → warning that it ships only if it compresses below the cap (extension-based estimate; stored files can still shrink), with guidance to recompress intentional large assets with xz/zstd or fetch them at start-up.
• Uncompressed size over the cap but compressible → similar warning, noting how much is already compressed and any payload that sits near the limit once tar headers count.
• --json gains archiveLimits with the default ceilings, labelled as such since build runs offline and the server's negotiated figures still win at deploy.