fix: correct README description of MISSION.md

[cyforkk]

Summary\n\nThis PR resolves #1006 by fixing the inconsistent description of MISSION.md in the README.\n\n## Changes\n\n- Changed 'draft project-establishment proposal' to 'founding mission of the established TLP'\n- Now consistent with line 48 and the status banner in MISSION.md\n\n## Verification\n\n- [x] Cross-references entry no longer calls MISSION.md a draft\n- [x] Consistent with line 48 and MISSION.md status banner

[potiuk]

LGTM — this corrects a genuinely stale line. MISSION.md has carried a banner describing itself as "the original establishment proposal, kept as a historical record of the project's founding mission" since Magpie was established as a TLP, while this README bullet still called it a draft project-establishment proposal. Minimal, targeted diff; CI green.

One heads-up so it isn't a surprise: #1040 changes this exact same line with equivalent intent. Yours was opened first and stays closer to the existing bullet structure, so this is the one I'm approving — a maintainer will close the other as a duplicate. Nothing for you to do here.

Smaller observations

• README.md:238 — established TLP is a little more clipped than the phrasing this README already uses for the same document a few lines up (line 48: "the founding mission of the Apache Magpie Top-Level Project, originally filed as its establishment proposal"). Aligning the two would be marginally more consistent, but it is not worth a round-trip on a one-line docs fix.

Thanks for the first contribution to Apache Magpie.

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