test(cli): bind the shipped agentGuide const to the flag-registry parity check

[suhaanthayyil]

https://entire.io/gh/entireio/entire-graph/trails/40

#81 bound AGENTS.md's claims to the real argument parsers and the help.go flag registry.
The artifact that actually ships is the agentGuide const in internal/cli/agents.go — it is
what agent-guide prints and what init-agents writes into a consuming project's
.entire/graph-agent.md. Nothing held it to that contract, so a parser or flag rename could
stay green in CI while every install carried a guide telling agents to use a flag that no
longer parses.

• Factored the existing test body into checkGuideClaims, with an origin label so a drift
  report still names the text that has to change. The AGENTS.md test keeps its assertions and
  its message wording.
• Added TestAgentGuideConstMatchesFlagRegistry, which runs the same contract over the const
  in-package, with no file read.
• Taught parseGuideClaims to treat 4-space/tab-indented Markdown code blocks as invocations,
  alongside fenced blocks. Without this the const contributes zero flag claims: it uses the
  indented form throughout, while AGENTS.md uses fences. AGENTS.md has no indented code
  lines, so this is a no-op for the existing test. Covered by
  TestParseGuideIndentedCodeClaims.

The const now contributes 9 command claims and 20 flag claims. Mutation-checked: inserting a
--bogus-mutant flag into the const's reference block fails
TestAgentGuideConstMatchesFlagRegistry on both the parser and the help-registry assertion.

$ go test ./internal/cli/ -run 'AgentGuide' -count=1
ok  	github.com/entireio/entire-graph/internal/cli	0.957s

$ go test ./internal/cli/ -count=1
ok  	github.com/entireio/entire-graph/internal/cli	3.812s

$ go vet ./internal/cli/
(clean)

Test-only change; no production code touched.