feat(cairnline): classify sidecar tool errors by structured code

[chicoxyzzy]

Summary

The Hecate Cairnline sidecar client previously detected a not-found tool-level error by substring-matching Cairnline's prose ("not found"). This change makes it read the machine-readable structuredContent.error.code that Cairnline now emits on a failed MCP tool call, and demotes the prose match to a transitional fallback used only when the sidecar returns no structured code (pre-contract builds).

Before / After

• Before: projectCairnlineSidecarToolErrorIsNotFound(result.Text) — substring match on prose.
• After: projectCairnlineSidecarToolErrorIsNotFound(result) — reads result.Result.StructuredContent → {error:{code,message}} as the primary signal (code == not_found), falling back to the legacy prose match only when no structured code is present.

HTTP mappings unchanged: not_found → ok=false → 404; every other code (or read failure) → 502.

What changed

• New projectCairnlineSidecarToolErrorCode extractor decodes structuredContent into {error:{code,message}}, guarding nil/empty/null/malformed JSON (returns "").
• projectCairnlineSidecarToolErrorIsNotFound now takes the full result and prefers the structured code.
• Legacy substring match kept as projectCairnlineSidecarToolErrorTextIsNotFound, clearly commented as a transitional fallback.
• All 4 call sites converted (projects.get, assignments.launch_packet, assignments.context, assistant.proposals.get).
• Added table-driven regression tests for the extractor and the classifier (structured not_found → 404, other codes → 502, empty-structured + prose "not found" fallback → 404, empty + unrelated prose → 502).

Notes

• Standalone — does NOT bump the cairnline module pin. It reads the structured structuredContent.error.code contract added in feat(mcp): emit structured tool-error codes cairnline#79 via a local wire constant (cairnlineToolErrorCodeNotFound = "not_found"), deliberately avoiding a pin bump: bumping cairnline past alpha.5 pulls in the unrelated Assignment.ExecutionRef string→struct refactor (cairnline#76) whose Hecate-side adaptation is owned by the in-flight feat(projects)!: structured execution refs, awaiting_approval, and context memory through the Cairnline bridge #832. Follow-up: once feat(agent-adapters): guide Claude Code ACP credential setup #79 is tagged and feat(projects)!: structured execution refs, awaiting_approval, and context memory through the Cairnline bridge #832's ExecutionRef adaptation lands, bump the pin and swap the local constant for cairnline.ErrorCodeNotFound.
• Localized to the error-classification helpers + 4 call sites; touches no bridge mapping functions, so it rebases cleanly over feat(projects)!: structured execution refs, awaiting_approval, and context memory through the Cairnline bridge #832.
• Caveat: cairnline assistant.apply signals partial-failure via its result body status fields, not structuredContent.error.code. Hecate's sidecar reads don't use apply, so there is no impact here.

Verification

• go build ./... — clean
• go vet ./internal/api/... ./internal/orchestrator/... — clean
• go test ./internal/api/... — ok
• GOCACHE="$(pwd)/.gocache" go test -race -timeout 10m ./internal/api/... ./internal/orchestrator/... — ok

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