refactor(document): give internal::Document the read-only defaults nine engines pasted

[andiwand]

🤖 Generated with Claude Code

Step 1 of #770, the order that issue suggests: the mechanical one, ten files, no behaviour change.

internal::Document now answers the read-only case itself:

bool Document::is_editable() const noexcept { return false; }
bool Document::is_savable(bool) const noexcept { return false; }
void Document::save(std::ostream &) const { throw UnsupportedOperation(); }
void Document::save(std::ostream &, const char *) const { throw UnsupportedOperation(); }

and the nine engines that had pasted exactly that — csv, iwork, markdown, rtf, the three oldms documents, and the ooxml spreadsheet and presentation documents — drop both the declarations and the definitions. odf and ooxml text keep their overrides, which are the only two with anything real to say.

AGENTS.md gains a clause under Adding / extending a document format so the next engine does not paste them back.

Two #include <odr/exceptions.hpp> became unused with the stubs gone and are dropped with them.

No consumer-visible change, so no CHANGELOG.md entry.

Verification

• odr and odr_test build clean.
• ./odr_test --gtest_filter='Document*:*Save*:*Edit*:*edit*:*save*' — 56 tests, all pass.