docs(plans): record the upload session cap plan's step MRs

Fills the MR column for the five steps of the upload session concurrency cap plan that have merge requests: step 1 is !2471 (merged), step 2 is !2408 (merged), step 4 is !2465 (merged), step 5 is !2462 (merged) and step 6 is !2463 (merged). Steps 2 and 5 are merged; the other three are open. Steps 3 and 7 to 12 keep empty cells.

The step ids come from each MR title's (upload-session-concurrency-cap plan: <step>/12) marker rather than from the titles' prose, and the denominator matches the table's twelve rows.

The table has a single writer by design, per guardrail 4 in AGENTS.md: parallel step MRs editing their own rows collide wherever the rows are adjacent lines of the same file, and this plan currently keeps three steps in flight together.

The legend the plan was missing

docs/plans/TEMPLATE.md carries a paragraph below the Status table defining what an empty cell means. This plan was written without it, so its nine empty cells had no definition in the file at all — and with five rows now filled, an undefined empty cell reads as "not started" rather than "not yet recorded". The paragraph is copied from the template verbatim, so the definition holds on every row: steps 7 to 12 have no MR yet, and step 3 is the one open gap between filled rows.

Testing

mise run lint:markdown passes, 254 files, 0 errors. No code changes.