feat(install): add `install --plan` machine-readable receipt (#388)

An installer that configures MCP entries, instruction files, Skills, and hooks
across many agents should let callers see what it will mutate before it does.
`--dry-run` already lists planned writes in human-readable form; this adds the
machine-readable `agent.install.plan.v1` JSON receipt the issue asked for.

`install --plan` runs the real install dispatch in record-only mode — a global
g_install_plan recorder that each write-site function appends to (at the same
point it would write) while all mutations are disabled — then emits JSON with
agents_detected, config_files_planned, instruction_files_planned,
hooks_planned, writes_started:false, network_after_install:false, and
next_safe_command. Because the plan is recorded on the actual install code
path, it cannot drift from real behavior. No config is written, no index
deleted, no network used.

cbm_build_install_plan_json is exposed for testing; the test asserts the
receipt content AND that building it creates no config files.