Problem

Phase 1 (#104558, landed as e68669373280) established the noUncheckedIndexedAccess ratchet lane over seven leaf packages. Fresh full-inventory probes show the remaining scope, with heavy transitive overlap because TypeScript applies the flag to every file in a program:

Structural facts: only leaf packages can gate via the ratchet lane; ui/, extensions/, scripts/, and memory-host-sdk all pull src/** transitively, so their lanes can only flip after src/ is clean.

Plan

1. Phase 2: extend the ratchet lane to all eleven remaining packages (only ai + agent-core need fixes; nine are already clean); complete packages/.
2. Phases 3a/3b: burn down src/ in reviewable chunks (ungated, regression-checked per PR).
3. Phase 3c: fix ui-own errors, then flip noUncheckedIndexedAccess: true in tsconfig.core.json + tsconfig.ui.json; memory-host-sdk becomes gated by the core flag.
4. Phase 4: extensions-own errors, flip tsconfig.extensions.json.
5. Phase 5: scripts-own errors, flip tsconfig.scripts.json.
6. Phase 6: re-probe test lanes with prod clean; fix or take a documented per-lane opt-out decision.
7. Endgame: move the flag to base tsconfig.json, remove per-lane flags, delete the ratchet lane and its routing.

Each phase also carries a bounded cleanup mandate in touched files (latent bugs, dead paths, closed-union tightening), with per-change rationale and net-LOC discipline.