Add Python preflight prompt to install.ps1; remove premature git-alias self-test

[J-MaFf]

Fixes #219

Changes

• install.ps1 now checks for a working Python interpreter (via the existing Resolve-Python) right after admin elevation and before any file is touched. Python only powers the git alias browser (rich/textual) — the core install works fine without it. A missing interpreter now surfaces as one clear [WARN] plus a Continue anyway? (y/n) prompt instead of three scattered warnings deep into the run. -Force auto-continues past the prompt (consistent with everywhere else -Force is used); declining exits 0 before any STEP runs or files are touched.
• Removed Initialize-Symlinks.ps1's STEP 2 git alias self-test. It ran before install.ps1 STEP 6 installs the rich dependency gitconfig_helper.py hard-requires at import time, so it structurally could never pass on a fresh Python install — it always printed [WARN] git alias command failed, even on runs that went on to succeed (STEP 7 re-runs the identical, correctly-ordered check later in the same run and is now the sole git alias verification).
• Updated CHANGELOG.md [Unreleased] with both changes.

Spec: specs/install-python-preflight.md (produced via the blueprint skill from a real install-run diagnosis in conversation, then implemented via the forge generator/evaluator loop).

Testing

• forge generator/evaluator loop: all 8 acceptance criteria (C1-C7, C-final) passed on round 1.
• Evaluator manually traced all four {-Force present/absent} x {Python found/not found} combinations against the spec's R3 and confirmed exact-match behavior (including exit code 0 and zero files touched on decline).
• Full tests/run-tests.ps1 Pester run: 221 passed / 3 failed / 18 not-run — the 3 failures are pre-existing (tests/gitconfig_helper.Tests.ps1), confirmed identical against a main baseline. Zero new failures.
• Did not run install.ps1 end-to-end on this machine (it requires admin elevation and would touch this machine's real ~/.gitconfig); verified via syntax parse + manual trace instead.

🤖 Generated with Claude Code

[J-MaFf]

Self-review

Re-read the full diff (CHANGELOG.md, STATUS.md, install.ps1, Initialize-Symlinks.ps1, specs/archive/install-python-preflight.md) after opening this PR.

• install.ps1's new preflight block sits strictly between the admin-elevation try/catch and the [STEP 0] banner, so it runs exactly once per invocation (in the elevated process) rather than once per elevation hop.
• It reuses the existing Resolve-Python from Functions.ps1 — no duplicate interpreter-probing logic introduced.
• Traced all four {-Force present/absent} × {Python found/not found} combinations by hand against the diff: decline (-Force absent, non-y answer) exits 0 before Cleanup-GitConfig.ps1 or any STEP runs, so nothing is touched on the user's home directory.
• Initialize-Symlinks.ps1's removed block was the only thing deleted — the scheduled-task prompt immediately above it and the trailing structure are untouched (confirmed via git diff).
• install.ps1 STEP 7's existing git alias/rich/textual verification is byte-identical to main — it remains the sole git alias self-test in the flow.
• Ran the full tests/run-tests.ps1 Pester suite: 221 passed / 3 failed / 18 not-run, with the 3 failures pre-existing on main (unrelated, in tests/gitconfig_helper.Tests.ps1) — zero new failures.
• Did not run install.ps1 end-to-end here (needs admin elevation and would touch this machine's real ~/.gitconfig) — verified via [Parser]::ParseFile (0 syntax errors) and manual trace instead.

No issues found; nothing to fix in a follow-up commit.