fix(core-agent): account for image context tokens

[CloudTianTian]

Summary

• count image content blocks in the shared session token estimator instead of pricing them at zero
• let image-heavy tool turns reach the active-checkpoint threshold before provider context overflow
• cover both direct token estimates and checkpoint triggering with regression tests

Source: Orkas commit f2c1c42d83f7b2914a247c6906adbe0ce35bfa57, adapted onto the current OrkasOpen main branch.

Testing

• npm run typecheck
• node scripts/run-tests.mjs run src/core-agent/test/session.test.ts — 76 passed
• node scripts/run-tests.mjs run src/core-agent/test — 32 files / 680 tests passed
• git diff --check
• selective OSS postcheck: no forbidden symbols/files, provider/API changes, orphan imports, UI/package violations, or TypeScript/renderer syntax failures

Local baseline limitations

• full npm test: 503 files / 7,371 tests passed; 3 environment-dependent tests failed because the local checkout lacks the OfficeCLI host binary and the existing video fixture tries to create /Users/userWorkSpace
• npm run test:resources could not start because the system Python does not have pytest installed
• the selective OSS postcheck still reports the pre-existing builtin-resource hash drift and full-sync coverage backlog; neither is changed by this two-file PR