docs: promote qtx README entrypoint

[Drswith]

Summary

• recommend qtx as the primary user-facing README entry point while keeping quantex explicitly equivalent
• add a first-class read-only no-install try-it-out section with validated package-manager command forms and current runtime caveats
• sync the Quantex skill wording and product-readme OpenSpec requirements with the accepted documentation contract

Linked Artifacts

• Issue: Promote qtx and read-only no-install usage in product-facing docs #75
• ADR: N/A
• OpenSpec: openspec/changes/promote-qtx-readme-surface/
• Discussion: [Decision] qtx 作为推荐入口与免安装试用在 README 的呈现 #69

Validation

• bun run memory:check
• bun run lint
• bun run typecheck
• bun run test (if behavior changed)
• Not run, explained below
• bun run openspec:validate

Release Intent

• Release: not applicable - docs/process/test-only change
• Release: patch - bug fix
• Release: minor - user-facing feature
• Release: major - breaking change

Docs Updated

• Not needed
• docs/...
• openspec/...
• Follow-up issue or OpenSpec change created instead

Scope Check

• I did not add a new ad hoc root-level Markdown file.
• I updated the relevant issue, ADR, spec, runbook, or captured the missing doc work as follow-up.
• I did not silently expand project scope without recording it explicitly.

Closure Check

• Working tree was clean after commit.
• Branch was pushed and this PR is the active delivery artifact.
• OpenSpec change is not needed, still active by design until merge, or already archived.
• Release is not applicable, delegated to release automation, or verified.

Notes

• bun run memory:check was not needed because this change does not alter project-memory rules or durable workflow artifacts beyond the OpenSpec/README updates included here.
• bun run test was not run because the implementation only changes README, skill wording, and OpenSpec documentation; no CLI behavior changed in source code.
• Follow-up Issue Keep Windows qtx.exe consistent with quantex.exe across self-upgrade and uninstall #76 tracks the Windows qtx.exe / quantex.exe consistency work that Discussion [Decision] qtx 作为推荐入口与免安装试用在 README 的呈现 #69 explicitly split out of this docs-scoped change.