fix(server): stop assignee updates bypassing the assign capability

[plusky]

update_issue_fields gated on Capability::Fields and banned only security, project, reporter — but assignee was not banned, so a caller granted fields but denied assign could reassign issues by sending the exact body assign_issue would have sent, and the Assign gate never ran. parent (re-parenting across hierarchies) and resolution (edit-screen resolve) had the same shape.

The banned set is now derived structurally: capability_owned_fields is an exhaustive match on Capability returning the fields each capability owns, so adding a future capability forces a compile-time revisit instead of relying on a hand-maintained string list. A new capabilities! macro generates Capability::ALL from the same variant list so the two cannot drift. DESIGN.md's I7 wording now names the "field whose mutation has its own capability" rule and justifies parent and resolution explicitly.

Invariants: I7 (no privileged-field smuggling), I9 (strictly tightening). The refusal is only reachable after deny_unless, so no I2 existence oracle.

Tests: this PR went through three adversarial review rounds. Rounds one and two proved the helper-level tests could not pin the fix (reverting the handler wiring left the suite green). Round three adds tests/server_mcp.rs — a real rmcp client over an in-process duplex transport with wiremock as Jira — asserting the refusal at the tool boundary with .expect(0) on the PUT endpoint, plus an allowed-edit control test proving refusals aren't vacuous. The final reviewer independently re-verified: gate reverted ⇒ boundary tests fail, helper tests stay green (the exact prior hole), full suite + cargo deny green. Dev-deps added: rmcp client feature and wiremock for the jirakeep crate (one-line lock diff, no new packages; production dependency graph byte-identical).

Process: AI-assisted — implemented and adversarially reviewed against docs/DESIGN.md by independent Claude agents (final verdict: approve, with empirical pre-fix-failure checks at the MCP boundary). Full verification suite green at the commit.