docs(instructions): use the graph to narrow scope, the source to verify

The generated instruction section told agents to ALWAYS use the graph
before Grep/Glob/Read and to fall back to file search only when the graph
did not cover the need. In practice that made models act on graph
summaries, skip the implementation, and produce shallower work. A zero
result can also mean "not indexed" rather than "does not exist".

Every platform instruction file now carries the same short guardrails:
narrow scope with the graph then read the source, read the implementation
and its tests before a non-trivial change, verify the exact source for
behavior, database, migration, retry, fallback, recovery and compatibility
code, prefer the source when the two disagree, and treat an empty result as
possibly unindexed. The intro and the "fall back only when" line were
tightened to pay for most of the space, so the injected section grows by 6
lines (38 to 44) for CLAUDE.md and 5 lines (46 to 51) for Copilot. The
intro and guardrails are shared constants so all 9 targets stay identical.

The four skill templates no longer say ALWAYS start with
get_minimal_context_tool, and each gained one line about reading the
implementation before changing code. Bundled skills/ regenerated to match.

CONTRIBUTING.md gains a platform target section stating what a new AI-tool
install target must include before review: official MCP config docs, the
exact path and schema, an entry through the existing PLATFORMS table,
preserved unrelated settings, byte-idempotent reinstall, working uninstall,
lifecycle tests matching the existing ones, and evidence from a real
released client. Requests no maintainer can test may be declined.

Fixes #314

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012fHfGDiZedoxjpKzanHri3