fix(update): preserve agent symlink destinations during skill updates

Why

Updates reuse detected-agent targeting from add -y, so an existing Hermes directory can gain links for skills installed only for Claude Code. Refresh shared content while preserving each skill's existing destinations.

Refs #1718.

What changed

• Refresh shared content without creating or restoring agent links in global and project updates. Only canonical-matching links identify shared installations.
• Warn about preserved redirects, show their targets, and print source- and scope-correct recovery guidance for a named shell. Unrelated inspection errors remain nonfatal when shared storage already exists.
• Retain explicit copy behavior and handle placement failures per skill, with isolated CLI regressions for link ownership, aliases, copies, notices, and batch continuation.

Scope

• Fix symlink-mode update targeting. Explicit add, removal behavior from #1786, and direct shared-directory visibility remain unchanged.
• No new flags or lock-schema changes. Broader copy-mode, exclusion, and well-known copy-metadata fixes remain outside scope.
• Independent copies beside shared content remain unchanged, except existing Eve stores.

Tradeoffs

An erased installation without copy metadata has no recoverable agent selection. When selected for reinstall, it receives shared content without guessed links. Shared refreshes refuse a canonical skill directory that is itself a symlink. Foreign links remain untouched; Eve flat links require manual review before reinstalling.

Blast Radius

This changes update placement and feedback. Filesystem inference preserves current destinations but cannot distinguish accidental deletion from intentional removal.

Verification

• The original eight CLI regressions failed before the targeting fix. Follow-up coverage produced 21 expected failures before these enhancements; the 91 destination cases now pass.
• Isolated Linux, Node 24.20.0 and pinned pnpm 10.17.1: build, 1,143 tests in 79 files, type checking, and formatting passed. The focused macOS run passed 176 tests in six files.
• Fixtures use temporary homes/projects, local Git, and loopback HTTP. POSIX recovery arguments were parsed through sh; PowerShell rendering was simulated. The upstream Windows and Node matrix still requires CI approval.