refactor(hooks/builtins): inline GetEnvironmentInfo + simplify package

[dgageot]

Move session.GetEnvironmentInfo (and its only filesystem helper, isGitRepo) out of pkg/session and into pkg/hooks/builtins, where the lone production caller lives. Then take a small simplification pass over the builtins package now that everything is colocated.

The function had a single production caller — add_environment_info.go in pkg/hooks/builtins — and a handful of unexported helpers (boolToYesNo, getOperatingSystem, getArchitecture, isGitRepo) that nothing else in pkg/session referenced. Inlining the whole graph drops a cross-package import and lets the builtin testsuite exercise the helpers directly as internal-package tests.

Commits

1. refactor(hooks/builtins): inline GetEnvironmentInfo and isGitRepo
   Move the function + its helpers + the related tests from pkg/session to pkg/hooks/builtins. Privatize on the way in (no external callers). Tests move with the code as internal-package tests so they can keep exercising the helpers.

2. refactor(hooks/builtins): simplify add_environment_info
   Drop the boolToYesNo helper (one call site → inline ternary). Collapse the three-arm getArchitecture switch (only amd64 was non-identity) into a one-line if. Rename helpers to read as nouns (environmentInfo / displayOS / displayArch). Drop TestBoolToYesNo (helper gone) and TestGetEnvironmentInfoIntegration (duplicated the table-driven test, only differing by using os.Getwd as the cwd, which isn't a meaningfully distinct code path). Env-block format and addEnvironmentInfo session_start contract are byte-identical.

3. refactor(hooks/builtins): consolidate isGitRepo tests into a single table
   Four single-case tests (TestIsGitRepo, TestIsGitRepoParent, TestInvalidGitFile, TestIsNotGitRepo) collapse into one table-driven TestIsGitRepo with six explicit cases (positive, parent-dir walkup, .git-is-a-file, missing/empty paths, etc.). Coverage is identical; the table makes the contract — what counts as "a git repo" — visible at a glance.

4. refactor(hooks/builtins): drop dead IsZero and inline turnStartContext
AgentDefaults.IsZero() had no production callers; the empty-config contract it documented is already enforced (and tested) directly via ApplyAgentDefaults returning nil. turnStartContext was a one-line wrapper used by exactly two callers, while addEnvironmentInfo was already calling hooks.NewAdditionalContextOutput directly — inlining surfaces every builtin's target event (turn_start vs session_start) at the call site rather than hiding it behind a same-package helper. Tighten the package doc accordingly.

Validation

• go build ./... ✅
• mise test ✅ (97 packages OK, 0 failures)
• mise lint ✅ (golangci-lint: 0 issues, custom lint: no offenses, go mod tidy clean)

Behavior preservation

• Public API: only IsZero was removed, and it had no production callers.
• The <env> block format injected at session_start is byte-identical (verified by the table-driven TestEnvironmentInfo).
• Auto-injection of builtins from agent flags (AddDate / AddEnvironmentInfo / AddPromptFiles) is unchanged.

Assisted-By: docker-agent