fix(code): apply SessionStart hook env to UI git/gh commands

[posthog]

Problem

The Commit and Create PR buttons run git/gh from the main process (via simple-git and execGh), and both were inheriting bare process.env. That ignored the env updates the Claude Agent SDK applies per session through SessionStart-style hooks — the SDK writes each hook's output to <CLAUDE_CONFIG_DIR>/session-env/<sessionId>/<event>-hook-<N>.sh and sources those files before each of its own bash tool commands.

Most visibly: when a user has the Secretive code-signing hook installed and launches PostHog Code from the Dock (so the parent inherits the default macOS launchd `SSH_AUTH_SOCK`), the agent can sign commits but the UI Commit / Create PR buttons cannot — the `SSH_AUTH_SOCK` repoint the hook performs never reaches the git subprocess they spawn.

Changes

Mirror the SDK's session-env behavior in the main process so UI-triggered git/gh sees the same env the agent does:

• `apps/code/src/main/services/session-env/loader.ts` — `loadSessionEnvOverrides(sessionId)` sources the SDK's `-hook-N.sh` files via `bash -c '... ; env -0'`, parses the result, and returns the diff against `process.env` (skipping shell internals like `BASH_*`, `SHLVL`, `PWD`).
• `AgentService.getSessionEnvForTask(taskId)` — resolves the most recent active session for a task and returns its overrides.
• `@posthog/git` — added optional `env?: Record<string, string>` to `execGh`, `ExecuteOptions`, and `GitSagaInput`. The operation-manager merges it on top of `getCleanEnv()` for the simple-git subprocess.
• `GitService.commit` / `createPr` — look up the session env when a `taskId` is present and thread it through commit, push, publish, and `gh pr create`. `createPr` resolves it once and passes it via an internal `envOverride` field on the commit options to avoid re-spawning bash for each step.

Test plan

• Loader unit tests pass (`pnpm --filter code test src/main/services/session-env/loader.test.ts`) — covers happy path, `printf %q` quoted values, multi-file ordering, ignored filenames, parent-env de-duping, missing dir, and bash failure.
• `pnpm typecheck` and `pnpm --filter code test` pass.
• Manual: with Secretive's `setup-code-signing.sh` SessionStart hook installed and the app launched from the Dock, click Commit on a task — commit succeeds and the signature is present (`git log --show-signature -1`).
• Manual: click Create PR in the same condition — the underlying commit signs cleanly and `gh pr create` runs with the hook env.
• Manual: tasks with no active session (e.g. before the user has prompted the agent) still commit/PR using bare `process.env`.

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