Escape values interpolated into graphql queries

[Thayorns]

Every query in lib/fbe/github_graph.rb dropped its arguments straight into the
query text, so an owner, a repository name, a branch or a pagination cursor
carrying a double quote silently rewrote the query instead of failing. Feeding
a") { id } repository(owner: "x as an owner turned one repository selection
into two fields, and graphql-ruby accepted it, so the caller got an answer about
something it never asked for.

Every such value now goes through a small Fbe::Graph::Literal object that
renders it as a quoted and escaped GraphQL string literal. The call sites lose
their hand-written quotes and read owner: #{Literal.new(owner)}, which keeps
the query text fixed no matter what the value carries. This covers owner, name,
branch, node id, the after: cursor in all four paginated queries, and the two
search(query:) strings in total_issues_created.

Two tests drive it: one feeds resolved_conversations an owner that tries to
open a second repository field, one feeds pull_requests_with_reviews a
cursor that tries to add an argument. Both assert the value arrives as a single
escaped literal, and both failed before the change.

allow_dynamic_queries stays on, since the multi-repo and multi-pull queries are
still assembled at runtime.

Closes #660

[Thayorns]

@yegor256 happy to clarify anything here, just say the word.

[Thayorns]

@yegor256 fixed here, take a look please

[yegor256]

This is the largest of the github_graph.rb changes and overlaps nearly every other open PR touching that file (#674, #669, #667, #670, #678, #671). Needs a human to pick a merge order or reconcile rather than merging blind. Assigning to @yegor256.

---

Generated by Claude Code

[Thayorns]

@yegor256 take a look please