Remove the recorded pre-run push outcome while preserving the push

[swerner]

What this removes

PreRunPushOutcome and GitContext.push_outcome were write-only freight: the manifest builder performed a useful best-effort push before a run and then serialized a report of what happened, but no production path ever read that report — every .push_outcome read in the workspace was a test. The type expanded into a public five-variant enum, five OpenAPI union arms, six generated TypeScript models, and constructor/fixture noise across the workspace.

This PR deletes the recorded outcome everywhere: canonical Rust (fabro-types), the manifest builder, OpenAPI (GitContext.push_outcome plus the PreRunPushOutcome union and its five arm schemas), the fabro-api type replacement and re-export, the generated TypeScript client models, and all fixtures/tests that constructed or asserted it. There is deliberately no replacement status type, flag, or message.

What this preserves

• The push itself. build_git_context still runs the best-effort push on the same path, now via a side-effect-only helper (push_manifest_branch_best_effort) with identical decision rules: no origin → skip; configured repository differs from the normalized origin → skip; branch already synced → skip; git probe errors still fall back to attempting the push; a needed push still uses remote origin, the detected branch, and GIT_TERMINAL_PROMPT=0.
• Non-fatal failure. A failed push still cannot fail manifest creation, and the discarded error (which may contain raw Git stderr) is still not logged or serialized.
• Useful git provenance. GitContext keeps origin_url, branch, optional sha, and dirty in canonical Rust, OpenAPI, events, manifests, and generated clients.

Test approach

Push behavior is now proven through observable repository state instead of the deleted enum: a local branch ahead of a temporary bare origin is pushed during manifest build (remote ref equals local HEAD afterward); a mismatched configured repository is not pushed (remote ref stays absent); a failing remote helper is invoked with prompting disabled while manifest creation still succeeds.

Compatibility

• Historical manifests and run.created events carrying a nested push_outcome remain readable through ordinary serde unknown-field tolerance, pinned by one regression test at the event boundary. No migration, event rewrite, or compatibility enum.
• This is a directional wire reduction: new servers/readers accept old JSON and ignore the removed field, but newly generated clients omit a field older servers required whenever GitContext was present, so new-client-to-old-server compatibility is intentionally not promised for this pre-1.0 contract.

Deliberately out of scope

• Input scalar coercion (a parallel branch owns that move; the only expected overlap is the fabro-types crate-root export list)
• The definition/intent request-contract migration
• Local target/workspace functionality
• Any redesign of push selection, credentials, retries, or Git error taxonomy

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