feat(install-targets): generic plan-diff retirement for all install targets

[kunaaaalcodes]

Summary

Extends the plan-diff retirement mechanism from OpenCode-only (Fmarzochi#1411) to every install target. When a file leaves an install plan, the copy an earlier install wrote is cleaned up on the next install/auto-update.

Problem

PR Fmarzochi#1411 added planRetirements to the OpenCode adapter, but every other install target (Cursor, Windsurf, Claude, Copilot, Kiro, etc.) had planRetirements: () => [] as a no-op default. Files copied to non-EGC-managed destinations accumulate silently when modules are deselected or plans change.

What Changed

• **\scripts/lib/install-targets/helpers.js**: Added \planGenericRetirements(), \collectCurrentlyCoveredDestinations(), and \isDestinationCovered(). Updated the default \planRetirements\ in \createInstallTargetAdapter\ to call \planGenericRetirements\ instead of returning [].
• **\scripts/lib/install-targets/registry.js**: Passes precomputed \operations\ into \planRetirements()\ to avoid redundant planning.
• **\scripts/lib/install-lifecycle.js**: Clarifying comment on \pruneOrphanedOperations\ explaining the split between source-gone pruning and plan-diff retirement.
• **\ ests/lib/install-targets.test.js**: 2 new tests — generic default retirement + directory scaffold shielding.
• *\CHANGELOG.md* + **\docs/ROADMAP.md**: Entries added.

Identity Rules

The same identity rules from Fmarzochi#1411 apply: only regular files, no linked ancestor, byte-identical to the source EGC copied, are actually deleted (enforced by \�pply.js's \isRetirableFile). Sources no longer in the repository are left in place (report-and-leave). Only \copy-file\ operations are diffed; \merge-json\ and \hook\ have no retirement counterpart yet (open follow-up).

Validation

• All 150 install-targets tests pass (2 new tests added)
• ESLint clean (0 errors)
• CHANGELOG.md and docs/ROADMAP.md follow existing format

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[kunaaaalcodes]

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