fix: Consolidate-Tasks discards date prefix and chronological order

[MarkMichaelis]

Problem

Consolidate-Tasks.ps1 strips the leading YYYY-MM-DD- date prefix from every
imported design-note / PRD filename via Get-FeatureSlug. The original date --
and therefore the chronological ordering of the artifacts -- is discarded and
recorded nowhere. For example a real run collapsed seven dated design notes:

docs/designs/2026-04-12-fix-setup-config-path-plan.md -> tasks/fix-setup-config-path-plan.md
docs/designs/2026-04-13-auth-consolidation-plan.md    -> tasks/auth-consolidation-plan.md
...

The tasks/ listing no longer sorts chronologically and the provenance date is lost.

Fix

1. Preserve a leading YYYY-MM-DD- prefix on the destination name so the
   tasks/ listing sorts chronologically (slug + kind suffix retained).
2. Add a Source Date column to tasks/MIGRATION.md (embedded date, else
   the file's last-write date) and sort manifest rows chronologically -- this
   captures order for sources with no date in the name (session plan.md).
3. Update tasks/README.md and the script help; add tests.

[MarkMichaelis]

Follow-up design question raised during PR #175 (deliberately out of scope there):

Newly created @prd / @plan artifacts are named tasks/<feature>-prd.md /
tasks/<feature>-plan.md -- the filename contains neither a date nor the
issue number, only the feature slug. Ordering/identity for new files lives in
the branch (feat/<issue#>-...), the issue, and git history.

The PR fixes the migration path (legacy artifacts now keep / synthesize a date
prefix). It does not change how new files are named, because adding a
date/issue prefix would break the agents' exact-path lookups
(prd.agent.md, plan.agent.md) and require switching them to glob matching --
a cross-cutting change to the upstream naming convention affecting all consumers.

Decision needed: keep new files as bare <feature> slugs (current), or add a
sortable prefix and update the agents to glob? Needs human sign-off.