docs: record CSRG-U5 as the active unit

[novelKR]

Records CodeSpace's CSRG-U5 as the active unit in DevGuard's ledger. It is bookkeeping, the same kind of change as #25 and #28, and is not part of U5's definition of done.

Why

The owner directed CSRG-U5 to start on 2026-10-09, together with merging CodeSpace's test-timing cleanup. Its predecessor is on CodeSpace main: CSRG-U4 merged as novelKR/CodeSpace#90 (3e5e3e8), recorded here by #29. The cleanup merged on 2026-10-11 as novelKR/CodeSpace#91 (470069c); it changes no pin, so CodeSpace still links this repository at 6e7e065. The post-merge CI of #91 is green: CI run 38150456452 and documentation run 38150456959. U5 started on the owner's Mac on 2026-10-11 (draft novelKR/CodeSpace#92). The ledger still records U5 as not-started.

Changes

File	Change
milestones.json	CSRG-U5 is active, the value #25 and #28 gave U3 and U4 when they started. units.as_of is 2026-10-11. U6 stays 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 U5 is the active unit
AGENTS.md	The dated current-state note says U5 is active
docs/planning/README.md, docs/ko/planning/README.md	The dated current-state note says U5 is active
docs/planning/milestones/CS-RG.md, docs/ko/planning/milestones/CS-RG.md	The dated current-state note says U5 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. U5 is not recorded as merged: that waits for its CodeSpace PR and the owner's exact-head approval.
• 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. The DG-1 qualified release stays 0.1.0-5daee5d-b3fa569e.

Verification

At b33994d, based on main at 79cb66c:

• 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 U5 as not-started again, which would be inaccurate while U5 is under way.