docs: record CSRG-U3 as the active unit

[novelKR]

Requested by Seongjae · project thread

Records the start of CodeSpace's CSRG-U3 in DevGuard's ledger. It is bookkeeping, the same kind of change as #22, and is not part of U3's definition of done.

Why

The owner directed U3 to start on 2026-10-07. U2 is merged in CodeSpace (novelKR/CodeSpace#84, ebed574) with green post-merge CI, and the ledger still records U3 as not-started with the units dated 2026-10-03.

Changes

File	Change
milestones.json	units.as_of is 2026-10-07 and CSRG-U3 is active, the value U2 had while it was the active unit. U4–U6 stay not-started. CS-RG stays in-progress, qualification not-run
docs/milestones.md	The CS-RG row and the dated note under the suspension notice say U3 is the active unit
AGENTS.md	The dated current-state note says U3 is active
docs/planning/README.md, docs/ko/planning/README.md	The dated current-state note says U3 is active
docs/planning/milestones/CS-RG.md, docs/ko/planning/milestones/CS-RG.md	The dated current-state note says U3 is the active unit

Korean counterparts reviewed; planning-readme and planning-milestones-cs-rg hashes recorded with scripts/check_docs.py record.

What does not change

• CS-RG is not marked implemented or qualified.
• The suspension of the CSRG-C00–C09 plan stays in force, and no historical text is rewritten: each change is the existing dated note or the living status row.
• No product code, contract, design revision, pin, dependency, CI policy or test changes.

Verification

At 9911112, based on main at 334472a:

• python3 scripts/ci_plan.py --base origin/main --head HEAD: profile affected, 8 normative paths, stages whitespace, source-contract and documentation, no Rust.
• python3 scripts/validate.py --stages whitespace,source-contract,documentation --diff-base origin/main: all three passed.
• python3 scripts/check_docs.py: passed, 18 reviewed pairs, 48 work units, 25 logical PR groups.

Rollback

Revert the merge (git revert -m 1 <merge commit>). The ledger then reports U3 as not-started again, which would be inaccurate while U3 is in progress.