feat: add OpenCode V2 adapter

[ousamabenyounes]

Add a separate OpenCode V2 plugin export that injects the shared Ponytail instructions through the V2 session context hook. Preserve persisted mode handling and the existing V1 adapter.

Test verification (RED → GREEN)

With the fix reverted, the new test fails (RED):

TAP version 13
# Subtest: V2 adapter registers a unique plugin and injects the persisted mode
not ok 1 - V2 adapter registers a unique plugin and injects the persisted mode
  ---
  duration_ms: 1.07946
  type: 'test'
  location: '/home/ousama/www/public/ousamabenyounes/ponytail/tests/opencode-v2-plugin.test.js:41:1'
  failureType: 'hookFailed'
  error: "Cannot find module '/home/ousama/www/public/ousamabenyounes/ponytail/.opencode/v2/ponytail.mjs' imported from /home/ousama/www/public/ousamabenyounes/ponytail/tests/opencode-v2-plugin.test.js"
  code: 'ERR_MODULE_NOT_FOUND'
  stack: |-
    finalizeResolution (node:internal/modules/esm/resolve:274:11)
    moduleResolve (node:internal/modules/esm/resolve:859:10)
    defaultResolve (node:internal/modules/esm/resolve:983:11)
    #cachedDefaultResolve (node:internal/modules/esm/loader:731:20)
    ModuleLoader.resolve (node:internal/modules/esm/loader:708:38)
    ModuleLoader.getModuleJobForImport (node:internal/modules/esm/loader:310:38)
    onImport.tracePromise.__proto__ (node:internal/modules/esm/loader:664:36)
    TracingChannel.tracePromise (node:diagnostics_channel:350:14)
    ModuleLoader.import (node:internal/modules/esm/loader:663:21)
    defaultImportModuleDynamicallyForScript (node:internal/modules/esm/utils:235:31)
  ...
1..1
# tests 1
# suites 0
# pass 0
# fail 1
# cancelled 0
# skipped 0
# todo 0
# duration_ms 87.499084

With the fix applied, the test passes (GREEN):

GREEN attempt 1/2 (exit 0)
TAP version 13
# Subtest: V2 adapter registers a unique plugin and injects the persisted mode
ok 1 - V2 adapter registers a unique plugin and injects the persisted mode
  ---
  duration_ms: 11.759577
  type: 'test'
  ...
1..1
# tests 1
# suites 0
# pass 1
# fail 0
# cancelled 0
# skipped 0
# todo 0
# duration_ms 134.866061

Full local suite

Command: node scripts/check-rule-copies.js && node scripts/check-versions.js && npm test

# tests 23
# suites 0
# pass 23
# fail 0
# cancelled 0
# skipped 0
# todo 0
# duration_ms 115.320577

> ponytail-mcp@4.9.0 test
> node --test ./test/*.test.js

TAP version 13
# Subtest: resolveMode keeps valid intensities
ok 1 - resolveMode keeps valid intensities
  ---
  duration_ms: 3.530473
  type: 'test'
  ...
# Subtest: resolveMode falls back to a runtime intensity for off/unknown/empty
ok 2 - resolveMode falls back to a runtime intensity for off/unknown/empty
  ---
  duration_ms: 1.174411
  type: 'test'
  ...
# Subtest: buildInstructions returns the ruleset tagged with the resolved mode
ok 3 - buildInstructions returns the ruleset tagged with the resolved mode
  ---
  duration_ms: 1.280801
  type: 'test'
  ...
1..3
# tests 3
# suites 0
# pass 3
# fail 0
# cancelled 0
# skipped 0
# todo 0
# duration_ms 111.474179

Fix #718