[FEATURE] Add `--dry-run | -d` mode for add/install/update

[Fcmam5]

Feature Description

Implement a real --dry-run mode for Skilleton so commands report planned changes without mutating manifest, lockfile, or install directories.

Motivation

Users need a safe planning mode before making lockfile/manifest/install changes, especially in CI and when reviewing dependency changes.

Use Case

• Who benefits: contributors reviewing skill updates, CI pipelines validating drift, teams enforcing controlled lockfile changes.
• What they gain: preview of lockfile additions/removals/updates and install actions before write/install.
• Frequency: common in CI and pre-commit workflows.

Detailed Description

--dry-run should:

• Work for add, install, and update.
• Produce a clear summary of intended actions:
  • manifest changes (add),
  • lockfile create/update/prune decisions,
  • skills that would be installed/reinstalled.
• Perform reads and resolution as needed to compute plan.
• Perform no writes and no filesystem installs/symlink changes.

Command specifics:

• add --dry-run:
  • parse and validate input,
  • show resulting manifest diff and lockfile/install actions,
  • show symlinks to be created (TBD in [FEATURE] Agent-aware skill symlinks #4)
  • do not write skilleton.json or skilleton.lock.json,
  • do not run actual install.
• install --dry-run:
  • show whether lockfile would be created/updated/pruned,
  • show which skills would be installed.
• update --dry-run:
  • show lockfile changes and changed skills to reinstall,
  • mode for add/install/update
  • skip actual writes/install.

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