What this adds

Companion CLI work for the epics epic in fangorn/anvil#118. Server-side already accepts `kind` and `parent` on issue create/update (fangorn/anvil ce8b969). This wires the CLI to use them.

`anvil issue create`

• `–epic` — sends kind=epic in the payload
• `–parent ` — sends parent in the payload; server atomically links via parent_of and auto-marks the parent as :epic
• The two flags are mutually exclusive (clap conflicts_with)
• `view` shows the kind when non-standard

`anvil epic` subcommand

``` anvil epic list list epics in the repo anvil epic view <#> title, state, progress, children table anvil epic children <#> bare children table anvil epic add-child link via parent_of anvil epic remove-child walk children to find link id, then delete anvil epic mark <#> promote to epic anvil epic mark <#> –unmark demote to standard ```

Cross-repo refs (`org/repo#5`) work everywhere, plus bare numbers (`5`) and same-repo refs (`#5`).

Tests

• 4 new unit tests for ref-matching helpers
• All 22 tests passing
• `cargo clippy` clean, `cargo fmt` clean

Part of fangorn/anvil#118 (epic) — child fangorn/anvil#112 (CLI).