fix(install): make bootstrap installer checksum verification fail closed

[jfischer-motivity]

Summary

Addresses recommendation #2 from the security review: make install verification fail closed.

The four first-install paths previously treated SHA-256 checksum verification as best-effort — if checksums.txt couldn't be fetched, the archive was missing from the list, or (in the shell installer) no hash tool was available, they silently skipped verification and installed the binary anyway. Only the in-binary update command failed closed. This contradicted the SHA-256 verification claim in SECURITY.md.

All four now abort rather than install an unverified binary when a positive checksum match can't be obtained. An explicit bypass — CBM_SKIP_CHECKSUM=1 (or --skip-checksum on the shell installers) — is provided for local development against unsigned test builds, with a warning.

Changes

File	Change
install.sh	Fail-closed checksum block; --skip-checksum flag + env bypass. Also || true on the grep so set -e/pipefail doesn't abort before the "not found" message
install.ps1	Same, PowerShell idiom. Also switched new error strings from em-dashes to ASCII hyphens (em-dashes inside double-quoted strings break parsing when the shipped UTF-8-no-BOM script runs via -File on Windows PowerShell 5.1)
pkg/npm/install.js	verifyChecksum now throws on any unverifiable state — it runs before the binary is copied, so no unverified binary is ever placed; documented why postinstall still exits 0 (keeps the bin shim linked; bin.js re-verifies on first run)
pkg/pypi/src/codebase_memory_mcp/_cli.py	_verify_checksum now sys.exits on any unverifiable state (this file was not flagged in the review but had the identical fail-open pattern)
SECURITY.md	Updated the checksum claim to cover all installers and document fail-closed behavior + bypass

Verification

Each installer was exercised end-to-end (not just syntax-checked):

• install.sh — 6 scenarios against a local release server: valid → verified & installed; mismatch / missing-checksums / archive-not-listed → aborted, no binary written; env & flag bypass → warn & install.
• install.ps1 — full AST parse + 4 scenarios via -File: all aborts fire with correct messages; bypass proceeds past verification.
• install.js — 6 cases with stubbed https: valid resolves; mismatch / 404 / network-error / not-listed all throw; bypass resolves.
• _cli.py — 5 cases with monkeypatched urlretrieve: same results.

Out of scope

Other review findings (Windows /api/process-kill, UI CSRF/auth, /api/browse scoping, pinned internal releases) are not addressed here — this PR is scoped to recommendation #2 only.

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