docs: fix premature M6.1 readiness checkboxes

[IanFrelinger]

Summary

M6.1 technical readiness checklist incorrectly marked M1-M5 as complete [x] even though all their exit criteria remain open [ ]. This misleads board/CEO review into thinking these milestones are already closed.

Changes

• Changed M1-M5 checkboxes in section M6.1 from [x] to [ ] to match actual milestone status
• No substantive roadmap changes—only checkbox honesty alignment

Testing

Docs-only change. Verified:

• M1 exit criteria (lines ~175-180): all [ ] unchecked
• M2 exit criteria (lines ~240-244): all [ ] unchecked
• M3 exit criteria (lines ~352-357): all [ ] unchecked
• M4 exit criteria (lines ~430-434): all [ ] unchecked
• M5 exit criteria (lines ~540-546): all [ ] unchecked
• M6.1 checklist (lines ~564-568): now correctly shows [ ] for M1-M5

Testing strategy (blast radius)

Docs-only change, no code modified.

• make kernel-coverage-gate (if touching src/Ashlar.Core.* or src/Ashlar.Infrastructure)
• make kernel-gate (if touching kernel hosting / pipeline / profiles)
• make test-prod-style (if touching production DI / API / routing)

Checklist

• make test passes locally (N/A - docs only)
• Documentation updated (if applicable)
• No TODO or NotImplementedException left unresolved
• Breaking changes are documented (none)

Release (only when this PR ships a versioned NuGet/GHCR release)

• Not a versioned release — skip