docs(progress): Phase 14 complete Phase 15 active (#537)

[DeliciousBuding]

Summary

• Mark Phase 14 complete ([P14.0] MASTER thin update Phase 13 complete / Phase 14 active #525–[P14.5] Settings residual inventory SectionId/surfaceMetadata #530 / PRs docs(progress): Phase 13 complete Phase 14 active (#525) #531–fix(ui): AgentsPage error StatusNotice residual (#529) #536)
• Activate Phase 15 residual hotspots (milestone 36, [P15.0] MASTER thin update Phase 14 complete / Phase 15 active #537–[P15.5] Repo hygiene after Phase 14 merges #542)
• Keep docs/progress/MASTER.md ≤150 lines

Test plan

• wc -l docs/progress/MASTER.md ≤150
• No trailing whitespace
• Phase 14 closed / Phase 15 pointers present

Closes #537

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