docs: add Prerequisites section to BUNDLE_GUIDE.md

[Brian Krabach (bkrabach)]

Summary

• Adds a Prerequisites section to docs/BUNDLE_GUIDE.md, documenting a convention that is followed in practice but currently absent from the spec.
• Covers: the bundle format has no post-install hook, the verbatim Prerequisites Check convention observed in two bundles (plus a closely related agent-level variant in two others), the userspace-vs-privileged-install boundary, and where to put the check (skill/agent/module body).
• Flags amplifier-bundle-browser-tester's ungated agent-browser install --with-deps as a known divergence, not a second sanctioned convention.

Evidence (verified independently against each repo, not taken on the request as-is)

• amplifier-bundle-digital-twin-universe and amplifier-bundle-gitea both carry the verbatim heading ## Prerequisites Check and sentence "If prerequisites are missing, report clearly and stop. Do not attempt workarounds." in skills/<name>/SKILL.md — confirmed.
• docs/BUNDLE_GUIDE.md has zero matches for prerequisit|post-install|install hook|system dep (case-insensitive) prior to this change — confirmed.
• amplifier-bundle-browser-tester self-installs agent-browser via npm install -g agent-browser, and on Linux also runs agent-browser install --with-deps (system packages) as a normal, ungated setup step in its bundle.md/README.md/context/browser-awareness.md — confirmed.
• amplifier bundle add (checked via --help against the installed CLI) takes only URI, --name, --app — no post-install hook exists in the format — confirmed.
• Correction to the originally reported evidence: amplifier-bundle-amplifier-tester does not carry the identical ## Prerequisites Check heading/sentence found in the other two. Its match (agents/setup-digital-twin.md) uses a different heading, ## Prerequisites Self-Check (REQUIRED), and slightly different wording, "If any prerequisite is missing..." — the same variant used by amplifier-bundle-browser-tester's agent files, not the digital-twin-universe/gitea SKILL.md wording. The PR documents both the verbatim SKILL.md-level convention and this agent-level variant rather than asserting three-way verbatim identity.

Test plan

• Verified all four source bundles directly (cloned, grepped) rather than trusting the summary
• Confirmed zero pre-existing matches in BUNDLE_GUIDE.md for the relevant terms
• Confirmed amplifier bundle add --help has no post-install hook option
• N/A — documentation-only change, no code paths to test

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