fix: Consolidate-Tasks preserves dates and chronological order

[MarkMichaelis]

Summary

Consolidate-Tasks.ps1 stripped the leading YYYY-MM-DD- prefix from every
imported artifact via Get-FeatureSlug, discarding the only chronological
ordering signal that legacy design notes / PRDs carried. The date was recorded
nowhere -- a real run collapsed seven dated design notes into undated slugs.

Changes

• Preserve an embedded YYYY-MM-DD- prefix on the destination name so the
  tasks/ listing sorts chronologically.
• Synthesize a date for undated sources via new -InsertDatePrefix switch
  (default on): most-recent git commit date, falling back to the file's
  last-write time. Pass -InsertDatePrefix:$false to opt out.
• Add a Source Date column to tasks/MIGRATION.md and sort rows
  chronologically.
• New helpers Get-DatePrefix, Get-GitDate, Get-SourceDate with unit
  • integration tests (43 passing). Updated comment-based help and
    tasks/README.md.

Scope decision (needs sign-off)

New @prd / @plan artifacts intentionally keep the bare <feature> slug
(no date, no issue number in the filename) so the agents can resolve them by
exact path. Their ordering/identity lives in the branch (feat/<issue#>-...),
issue, and git history -- not the filename. Date-prefixing new files would
require switching the agents to glob lookups, a cross-cutting change to the
upstream naming convention affecting all consumers. Deliberately not done
here. See the open question below.

Open question

Should newly created PRDs/plans also carry a sortable prefix (date or issue
number) in the filename, accepting the agent-lookup change? Flagging for a human
decision rather than guessing.

Test

Invoke-Pester -Path .\Consolidate-Tasks.Tests.ps1

Closes #174

[MarkMichaelis]

Decision: revive, but expect rework rather than a rebase.

The bug is still live. Consolidate-Specs.ps1 strips the date prefix today:

$stem = $stem -replace "^\d{4}-\d{2}-\d{2}-", ""

Nothing else records it, so the ordering signal is still being destroyed — the PR body's "seven dated design notes collapsed into undated slugs" remains reproducible.

The friction is that the script was renamed (Consolidate-Tasks.ps1 → Consolidate-Specs.ps1) since this branch was cut, so this will not apply cleanly. Three files, one regex plus tests: porting the fix is cheaper than rebasing the branch.

Assessed alongside #250 (revive) and #161 (closed). Recorded here because the decision was made in a session that ended before acting on it.

[MarkMichaelis]

Flagging for closure — this PR cannot be rebased, because all three files it edits no longer exist:

File	Status
Consolidate-Tasks.ps1	renamed to Consolidate-Specs.ps1 (#184)
Consolidate-Tasks.Tests.ps1	same rename
tasks/README.md	tasks/ retired for docs/specs + docs/designs (#182)

Opened 2026-05-31; both structural changes landed after it. There is nothing to rebase onto.

The issue it fixes, #174, may still be valid — Consolidate-Specs.ps1:103 does a bare Sort-Object with no key, which would sort lexically and is consistent with the reported "discards chronological order" symptom. But that is a suspicion, not a verified finding: #174 was written against the pre-rename script and nobody has confirmed the defect survived.

Suggested: close this PR as superseded by the rename, leave #174 open, and re-confirm the bug reproduces against Consolidate-Specs.ps1 before writing a fix. Not closing it here — that is the owner's call.

Tracked in #465.