Adopt standard docs/ spec structure (docs/specs + docs/designs), retire tasks/ convention

[MarkMichaelis]

Summary

Replaces the bespoke tasks/ "durable spec archive" convention with the
industry-standard docs/ structure across the whole framework. The goal is a
durable requirements corpus you could hand to a fresh AI to rebuild the app
from scratch -- docs/specs/ (the what) + docs/designs/ (the how) reads
as the standard convention, whereas tasks/ reads as transient work items.

Mapping

Old	New
tasks/<issue#>-<slug>-prd.md	docs/specs/<issue#>-<slug>-prd.md
tasks/<issue#>-<slug>-plan.md	docs/designs/<issue#>-<slug>-plan.md
tasks/README.md	docs/README.md (consumer-owned, same-name scaffold)
tasks/MIGRATION.md	docs/MIGRATION.md

Changes

• Pull-SDLC.ai.ps1 -- TemplateScaffoldMap, UpstreamManagedPaths and
  AlwaysLocalPaths now carry docs/README.md, docs/specs/, docs/designs/.
  Carve-out comments updated.
• Consolidate-Tasks.ps1 -- inverted: consolidates legacy locations
  (tasks/*, docs/prd/*, root PRD.md/plan.md/IMPLEMENTATION_PLAN.md,
  copilot/claude sessions) into docs/specs/ and docs/designs/; manifest
  at docs/MIGRATION.md. Filename intentionally kept to avoid churning the
  meta-script lists.
• Agents (@prd, @plan, @dev-loop), README.md, .gitignore
  updated to the new paths.
• git mv tasks/README.md docs/README.md + content rewrite; removed
  tasks/.gitkeep and the empty tasks/ dir.

Testing

• Invoke-Pester Consolidate-Tasks.Tests.ps1 -- 33 passed, 0 failed
• Invoke-Pester Pull-SDLC.ai.Tests.ps1 -- 190 passed, 0 failed
• Behavior-first: tests updated to encode the new behavior before the scripts.
• CRLF/LF endings preserved per-file to keep diffs minimal.

Closes #182