feat: add install-info command and buildInstallSnippet API

[steve-calvert-glean]

Summary

Implements the install-snippet feature planned alongside the target registry migration — every PluginTargetDefinition already carries an installSnippet (populated as each target migrated in #14/#15/#16), so this wires it up to a public API and CLI command rather than introducing new per-target data.

• src/install-snippet.ts: buildInstallSnippet, getInstallSnippetCitation, getSupportedInstallTargets, getUnsupportedInstallTargets — thin wrappers around each target's registry entry.
• New pluginpack install-info [--target <t>] [--json] CLI command, defaulting to every configured target (matching build's pattern). Resolves each plugin's path via that target's own resolvePluginPath for accuracy — antigravity's snippet is the one that actually consumes pluginPath.
• targets.<name>.repository config field (falls back to metadata.repository, mirroring updateCheck.repository's existing fallback) — "which repo does this target's output live in."
• Exported from the package entry (buildInstallSnippet, getSupportedInstallTargets, getUnsupportedInstallTargets, plus Citation/InstallParams/InstallSnippet types) for programmatic use.
• README: new "Install Snippet" section, a Configuration Reference row, and Programmatic API table rows. CONFORMANCE.md: an "Install-snippet facts" table with each target's doc citation.

Test plan

• npm run typecheck
• npm run lint
• npm test (76/76 — tests/install-snippet.test.ts locks in the exact snippet text per target; tests/conformance.test.ts adds CLI end-to-end coverage — default, --target, --json, missing-repository error — via the real built binary)
• npm run build
• node dist/cli.js docs --check
• Manually smoke-tested install-info against a 5-target fixture in /tmp (all 5 snippets print correctly; --target/--json/missing-repo error all behave as expected)

Stacking

Based on refactor/delete-legacy-target-adapters (#17). Merge in order: #14 → #15 → #16 → #17 → this.

[glean-it]

Issue: Potential TypeError: buildInstallSnippet indexes target registry without handling partial registry entries. If registry[target] is undefined (e.g., targets not yet migrated to the registry), accessing .installSnippet will throw at runtime when CLI install-info or library callers pass such a target.

Suggested fix: Mirror the adapters' partial-registry pattern: guard registry[target] and fall back to a legacy install snippet definition or return a structured userConfigurable:false result with a clear reason. For example: const def = registry[target]?.installSnippet; if (!def || !def.userConfigurable || !def.build) return { userConfigurable: false, reason: def?.unsupportedReason ?? ${target} has no install command or URL. }; otherwise return { userConfigurable: true, ...def.build(params) }. Alternatively, ensure all TargetName variants are present in the registry before using it here.

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