feat(compat): adopt quantcli/common/compat/formats as second consumer

[Terastar-Paperclip]

Summary

Onboards liftoff-export-cli as the second machine-attested consumer of quantcli/common/compat, mirroring the crono pattern. Closes the liftoff side of QUA-16.

What

• compat_contract_test.go (build tag compat) — runs compat/formats.RunContract against the built binary via LIFTOFF_EXPORT_BIN.
• .github/workflows/ci.yml — adds two jobs:
  • go vet + test (parity with crono's default job).
  • compat (CONTRACT machine-attestation) — builds the binary, runs go mod tidy, then go test -tags=compat -run TestContractFormats ./....
• go.mod — pins github.com/quantcli/common/compat v0.0.0-…-de9219e03369 (merged-main pseudo-version, post-QUA-15 + nested-subcommand + SkipDataPath).

Surface

Subcommands targeted (the four --format-bearing leaves without a required positional):

• workouts list
• workouts stats
• bodyweights list
• bodyweights stats

workouts show is intentionally excluded because its required <date> positional would surface "missing arg" rather than format-flag failure in UnknownFormatFails.

Runner config

• SupportedFormats: [markdown, json] — CSV is not yet wired, so CSVHasHeader skips with a named reason.
• SkipDataPath: true — liftoff's data path requires a stored OAuth token at ~/.config/liftoff-export/auth.json which CI does not provision; the parse-level subtests (HelpDocumentsFormatFlag, UnknownFormatFails, FlagValidationIsHermetic) still attest §4.

Local verification

$ go build -o /tmp/liftoff-export .
$ LIFTOFF_EXPORT_BIN=/tmp/liftoff-export go test -tags=compat -run TestContractFormats ./...
ok  github.com/quantcli/liftoff-export-cli  0.031s

All four subcommand groups: 3 parse-level subtests PASS, 3 data-path subtests SKIP (SkipDataPath).

Test plan

• go vet + test CI job green.
• compat (CONTRACT machine-attestation) CI job green.
• Drift later (e.g. removing --format from a leaf) surfaces as a named subtest failure.

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