feat: background-agent model display, v1 wire 1.5 support, AGENTS.md map trim

[zicochaos]

Problem

Three independent items:

1. Background subagent entries (coder agent started in background) never showed which model the subagent is bound to.
2. Resuming a v2-written session (wire protocol 1.5) in the default v1 TUI printed: Warning: Session wire protocol version 1.5 is newer than the current version 1.4. Records will be replayed without migration.
3. The root AGENTS.md grew to 25 KB — pushing the merged AGENTS.md prompt context to 32.8 KB, past the engine's recommended 32 KB budget — mostly because ## Project Map entries became per-package essays (17.7 KB of the file).

What changed

One commit per item:

• feat(tui): BackgroundAgentMetadata gains model, populated from the subagent.spawned event's bound alias via the existing modelDisplayName resolution, and rendered in the headline — coder agent (GLM 5.2) started in background. v2-only (v1's spawned event carries no model; v1 entries render unchanged). +2 formatter tests.
• fix(agent-core): ports the engine-agnostic wire 1.4→1.5 migration from agent-core-v2 (backfills the goal wall-clock anchor from the record timestamp) into agent-core and bumps AGENT_WIRE_PROTOCOL_VERSION to '1.5', matching upstream v2. The 1.4→1.5 delta is one additive field, so v1 replay of v2-written sessions was already safe — this silences the warning and migrates genuine 1.4 files properly. +migration test (v2 suite's cases); the v1.4 test's snapshot now pins the literal 1.4 (the scrubber only masks the current version).
• docs: ## Project Map 17.7 KB → 2.9 KB (root file 25.2 KB → 10.3 KB). Detail moved, not deleted: new packages/kap-server/AGENTS.md, packages/transcript/AGENTS.md, packages/minidb/AGENTS.md, packages/tree-sitter-bash/AGENTS.md, apps/kimi-inspect/AGENTS.md; two facts merged into packages/klient/AGENTS.md. Root keeps 1–2-sentence pointers per package; all other root sections byte-identical. Also drops one stale fact (pnpm dev:v1 script no longer exists).

Verification

• TUI: 2488 passed; agent-core: 4139 passed (4 pre-existing /bin/bash NixOS env failures, unrelated); records/migration suites 52/52
• oxlint clean on touched files; tsc --noEmit clean in kimi-code and agent-core
• Root diff confined to a single Project Map hunk; 20 distinctive tokens from the moved prose spot-checked to resolve in exactly one package file each

Note: the condensed root AGENTS.md is an upstream-synced file — future upstream edits to Project Map entries may need conflict resolution against the pointer entries.