docs: add M11 collaborator execution plan

[BerryUIKI]

Summary

• add the active M11 Portfolio Hardening and Release Candidate milestone
• define seven strictly sequential child-agent tasks with objectives, acceptance criteria, involved files, branch types, and verification checklists
• synchronize README, milestone, delivery, i18n, Option, Goose, and Portfolio documentation with the current completed M0-M10 baseline
• preserve the original M8-M10 task queue as historical delivery evidence
• correct Portfolio contributor instructions to use focused branches from origin/dev and target dev
• make the existing PerformanceTab data fixture date-independent so the rolling one-month test remains deterministic

Scope

The planned deliverables are documentation only. The PR also contains one authorized test-only CI repair after the existing fixed-date PerformanceTab fixture aged outside its rolling one-month window. No production runtime behavior is changed.

This PR does not implement Portfolio cleanup, performance optimization, packaging, release publication, live broker connectivity, FIRE planning, trading, or autonomous investment decisions.

Verification

• custom structural validation: 7 M11 tasks, all five required task fields, approved branch patterns, and origin/dev to dev targeting
• documentation-only planning-path validation across 22 documentation files
• added-content English check
• local Markdown link validation
• milestone registry reference validation for i18n, Option, Goose, and M11 supporting documents
• selected Prettier checks passed for the new execution plan and canonical actively edited indexes
• focused PerformanceTab test passed: 5 tests
• complete Vitest suite passed: 64 files, 516 tests
• git diff --check passed

Application lint, typecheck, and Rust checks passed in the original workflow run before the date-sensitive frontend test failed. The pushed repair changes only frontend test fixture dates and triggers the complete GitHub checks again.

Execution note

After merge, assign M11-01 only. Do not start a later task until its predecessor is merged into dev and verification evidence is recorded.