Make truncation marker honest before the size-cap blob spill

[TheGreatAxios]

Summary

Tool results are truncated to MAX_RESULT_CHARS (80,000 chars) by src/plugins/result-truncation-plugin.ts in the posix chain (src/agent/posix-tool-plugins.ts) and the MCP runner (src/mcp/plugin.ts) before the reactor's size-cap transform runs (vendor/intx-inference/src/transforms/size-cap.ts). For an output over 80k, the size-cap transform spills the already-truncated text as the blob and tells the model "Full output available at tool-output:///{id} — use read_file with that URI to see the rest." That promise is false: the remainder was discarded upstream and the blob only contains the same truncated text. The model reads the blob, finds it cut in the same place, and re-runs the command — an observed loop shape in production traces.

vendor/ is edit-locked, so this fixes it at the source: the truncation marker in result-truncation-plugin.ts now tells the truth up front, so the lie doesn't get a chance to survive the size-cap spill.

Tradeoff

Kept the honest-but-terse wording rather than a fuller explanation, to stay under the MAX_RESULT_CHARS + 200 char allowance asserted by the real-chain ripgrep integration test (tests/unit/ripgrep-plugin.test.ts). The marker text is:

[output truncated at 80,000 chars — {n} chars discarded, NOT retrievable (no tool-output URI has them; re-running gives the same cut). Use offset/limit or a narrower query.]

This sacrifices some guidance detail for staying inside that budget, but the load-bearing part — "NOT retrievable" / "re-running gives the same cut" — is what stops the retry loop, so it comes first.

Test plan

• src/plugins/result-truncation-plugin.test.ts (new): asserts the marker never promises a retrievable remainder, and that it survives a real createSizeCapTransform spill (blob content ends with the marker, inline output still points at tool-output:///{id})
• tests/unit/ripgrep-plugin.test.ts: existing real-chain oversized-grep tests still pass within MAX_RESULT_CHARS + 200
• bunx prettier --check / bunx eslint on touched files
• bun run typecheck, bun run build
• bun test ./src ./tests ./evals (5116 pass, 1 pre-existing unrelated failure in src/agent/lsp-availability.test.ts present on main with no node_modules — not touched by this change)

Fixes CL-6908

[linear-code]

CL-6908