feat(threats): allow editing threat titles, keep category rule-owned

[Hacks4Snacks]

feat(threats): allow editing threat titles, keep category rule-owned

What changed

tmforge threats --edit and the Studio threats panel can now set a threat's
title. --title works on any threat; --category is accepted only on manually
authored threats and is refused with an explanation on generated ones.

• CLI: --title / --category on threats --edit
• MCP: title / category params on edit_threat
• Studio: title input; category select disabled on generated threats, with
  inline text saying why
• Clearing a title on a generated threat restores the rule's original wording
• Clearing a title on a manual threat is refused (nothing to fall back to)

Why

Rule wording is precise about a class of problem, not about your system.
Reviewers were rewording threats outside the tool and losing the edit on the
next run (THR-001).

Design note: title vs category

These sit on opposite sides of an ownership line, deliberately.

• Title is judgement — author-owned. The rule still fires, the finding id is
  unchanged, and the edit is reversible.
• Category is detection — rule-owned. Editing it on a generated threat is
  refused, not ignored, in the CLI, the authoring facade, and the UI alike.
  Silently dropping it would leave an author believing they had recorded
  something.

How to test

dotnet build dirs.proj -p:Platform=x64 -p:BuildStudio=false
dotnet test  test/dirs.proj -p:Platform=x64 -p:BuildStudio=false

Manual, against examples/webshop.tm7:

tmforge list threats  <model>                      # pick a generated threat id
tmforge threats <model> --edit <id> --title "Audit log can be tampered with"
tmforge threats <model> --write                    # title survives regeneration
tmforge convert <model> out.json && tmforge convert out.json rt.tm7
tmforge list threats rt.tm7                        # title survives round trip
tmforge threats <model> --edit <id> --category Tampering   # refused, exit 1
tmforge threats <model> --edit <id> --title ""     # restores the rule wording

Risk

GeneratedDefaultTitle is persisted per generated threat, which roughly doubles
per-threat title bytes (~5% file growth on a 76-threat model). Accepted for
parity with the existing priority path and because it is what makes an
externally-edited title survive.