docs(AGENTS): document project-board membership + status workflow; drop closed V1.5 board

[cliffhall]

Problem

AGENTS.md's Issue-driven Work Style section describes board interaction only in prose ("move the item to In Progress / In Review") and gives concrete, copy-pasteable `gh` commands only for the version label (`gh issue create --label v2`). As a result, agents reliably:

• apply the `v2` label (documented, one flag) but
• fail to add the issue/PR to the V2 board (Add tab and approval flow for server -> client sampling #28) and fail to set its Status (undocumented, multi-step, needs project/field/option IDs).

Labeling and board membership are two different things — a label is a repo tag; the board is a separate org project. Following the single concrete command in the doc produces a labeled-but-not-on-board item.

Also wrong / stale

• Status values are documented as "In Progress" / "In Review"; the real board options are lowercase "In progress" / "In review", plus `Backlog`, `Building CLI / TUI / CORE`, `Building Web`, `Done`.
• The V1.5 board (Multiline string inputs to tools isn’t supported #39) is still listed but is closed/done after the v1.5→v2 merge completed. It should be removed.

Proposed fix

Update AGENTS.md to:

1. Remove the V1.5 board (Multiline string inputs to tools isn’t supported #39) from the Project Boards list (and de-emphasize v1.5 work-style references as appropriate).
2. Add explicit, copy-pasteable `gh` recipes for adding an issue/PR to the V2 board and setting Status, including the V2 board's field/option IDs:
   • Project Add tab and approval flow for server -> client sampling #28, owner `modelcontextprotocol`
   • Status field: `PVTSSF_lADOCt2Azc4BJVxtzg5iI8c`
   • Add: `gh project item-add 28 --owner modelcontextprotocol --url `
   • Set status: `gh project item-edit --project-id PVT_kwDOCt2Azc4BJVxt --id --field-id PVTSSF_lADOCt2Azc4BJVxtzg5iI8c --single-select-option-id `
3. Correct the Status names to match the real board.
4. Make it unambiguous that creating an issue or PR for v2 work is incomplete until it is on board Add tab and approval flow for server -> client sampling #28 with a Status set (not just labeled).

[cliffhall]

Merged via #1555 into v2/main (squash d5d80d3). Closing manually since closing keywords don't auto-fire on the non-default v2/main branch.