psake 5.x migration: bump the pin to 5.0.4 and record the behavior change

[tablackburn]

Part of #120 (Phase 2 — psake 5.x bump). Graduated from "Not yet specified" by the #155 spike,
which found the migration is a single small PR, not a chain: the consumer-facing
PowerShellBuild/psakeFile.ps1 needs no changes at all.

Scope

• Bump psake from '4.9.0' to '5.0.4' in requirements.psd1
• Pick up 5ec9a01 from spike/psake-5x
  — the one-line Set-BuildEnvironment guard in tests/Manifest.tests.ps1, without which the
  suite loses 12 tests under psake 5.x (full analysis in psake 5.x spike: assess breakage under psake 5.0.4 (clean bump, no extras) #155)
• Migration-guide entry and changelog entry — dropped 2026-08-20. This change is
  not user-facing: requirements.psd1 is this repository's own build toolchain, and the
  manifest's RequiredModules floor stays at psake 4.9.0, so upgrading PowerShellBuild
  0.8.x → 1.0.0 does not move anyone to psake 5.x. The psake 5.x behavior differences are
  triggered by upgrading psake, and psake's own v4 → v5 guide is their home. If a later
  change raises the manifest floor to psake 5.x, that change adds the migration entry.

Already verified in #155

Full suite is 428 passed / 0 failed under psake 5.0.4 with the guard applied — identical to
the 4.9.1 baseline — and 428/0 under 4.9.1 with the same guard, so the fix is not version-specific.
All four breaking changes in psake/psake docs/migration-v4-to-v5.md
were checked and none apply.

Watch for

• CI runs the matrix this spike could not: Windows PowerShell 5.1 and macOS/Linux pwsh. The spike
  ran on pwsh 7.6.5 on Windows only. psake 5.x on Windows PowerShell 5.1 is unverified — if it
  fails there, that is a scope decision for [Tracking] PowerShellBuild v1.0.0 roadmap #120, not something to work around in the PR.
• Version 5 declaration and Properties @{} hashtable syntax are available in 5.x but are
  not part of this bump. Adopting them is a separate, optional change.

Done when

requirements.psd1 is on psake 5.0.4, CI is green on every leg, and the migration guide and
changelog record the PsakeBuildResult behavior change.

[tablackburn]

Implemented in #162 (branch feat/psake-5x-migration).

• requirements.psd1: psake 4.9.0 → 5.0.4
• Carries the Set-BuildEnvironment guard from the spike (5ec9a01, cherry-picked)
• Migration-guide entry "psake 5.x is now the tested build toolchain" covering both consumer-visible behaviors, with detection commands
• Changelog entry

Verified 428 passed / 0 failed under both psake 4.9.1 and 5.0.4, each with an isolated module root so exactly one version was resolvable (including for Start-Job children).

Manifest RequiredModules deliberately stays at psake 4.9.0 — the task definitions run on both majors, so raising the floor would force an upgrade for no functional gain, matching the Pester decision in #120.

Two things left for CI rather than local verification, both flagged in the PR:

1. Windows PowerShell 5.1 — the spike and this PR ran on pwsh 7.6.5/Windows only. Tracked as open fog on [Tracking] PowerShellBuild v1.0.0 roadmap #120.
2. psake-4 compatibility is no longer continuously tested — the manifest claims ≥ 4.9.0 but CI now runs only 5.0.4. Raised on [Tracking] PowerShellBuild v1.0.0 roadmap #120 as fog rather than decided here.

This ticket closes when #162 merges.