fix(cli): hint when structured tool content is hidden

[PLASMA-FR]

tools-call and tasks-result now show a concise --json hint when human output hides structuredContent. Existing duplicate-text filtering is retained; regression coverage, README, agent skill, and changelog are updated.

Reproduced on eb954d6 with a local stdio tool returning a summary plus serialized structured data: human output showed only the summary; --json retained both fields.

Validation:

• pnpm exec vitest run test/unit/cli/output.test.ts: 189 passed, including checks that failed on the original formatter.
• pnpm run lint (0 errors, 6 existing warnings), pnpm run build, and pnpm run check:reference passed.
• ./test/e2e/run.sh --no-build --parallel 3 basic/help.test.sh basic/human-output.test.sh basic/output-invariants.test.sh passed; local CLI JSON/verbose and skill checks passed under Node and Bun.
• pnpm test: 1,055 unit tests; E2E Node legacy 51 passed/1 skipped, Node modern 47/5, Bun legacy 51/1. All skips are existing protocol-specific exclusions.

Fixes #346

[PLASMA-FR]

The CI and PR toolkit workflows are still awaiting first-time contributor approval and have not created any jobs. When convenient, could a maintainer approve those workflow runs? The local validation and exact commands are recorded in the PR body; I will address any resulting failures.