README: add a beginner-facing "Using a skill" section

[eeholmes]

Closes #7

The README documented where skill directories go for each agent, but not how
to use one. Adds a Using a skill section before Installation, aimed at
people who have not used Agent Skills before.

Covers:

• what a skill is, and that it activates from the task description rather than
  a command — the most common beginner confusion;
• copy-pasteable install for the personal and project cases, symlinking into a
  catalog clone so git pull updates every install;
• the restart requirement, since skills are discovered at session start and a
  mid-session install appears to do nothing;
• how to verify, and a short "it did not trigger" fallback.

Includes mkdir -p in the install commands because the skills directory
usually does not exist yet and ln -s fails confusingly without it.

The existing Installation section, its per-agent paths, and its warning that
upstream locations change are unchanged.

Checklist

• The relevant issue is linked.
• The maturity level is appropriate. (no skill changed)
• Each changed skill has a valid SKILL.md. (no skill changed)
• Core instructions are concise and references are used appropriately.
• Bundled scripts are tested, or no scripts changed. (no scripts)
• Evaluations are added or updated when behavior changes. (no behavior change)
• Current official documentation was checked where relevant.
• No secrets or sensitive information are included.
• The README catalog is updated if a skill was added, renamed, stabilized,
  deprecated, or removed. (no catalog change)

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