CLI safety guard, Dockerized E2E, env docs, gm_cli_ctx_t

[flyingrobots]

This PR does the following:\n\n- fix(include): guard time conversion macros in constants_internal.h\n- fix(output): add EOF newlines in include/gitmind/output.h and apps/cli/output.c\n- chore(cli): normalize includes; add inc_cli in Meson; adjust CLI include paths\n- feat(cli): introduce gm_cli_ctx_t; pass explicit output context; remove global\n- feat(cli): libgit2-based safety guard; add GITMIND_SAFETY override; URL helper\n- test(core): add URL safety matcher unit test; register in Meson\n- docs(ops): document environment variables and link from README/docs index; note runtime safety\n- ci(e2e): run E2E inside CI Docker image for parity and isolation\n- fix(core/journal): temporary CBOR decode wrappers for CLI until attributed decoder lands\n\nBuild & test:\n- Built in CI Docker image with clang-20; all unit tests pass (including new safety test).\n- E2E now runs inside the CI Docker image.\n\nNotes:\n- The safety guard only blocks if a remote URL strictly matches neuroglyph/git-mind(.git).\n- is available to bypass if ever needed.\n\n

Summary by CodeRabbit

• New Features

  • New standalone CLI with a safety guard preventing accidental runs in the source repo (can be overridden); unified CLI output handling and improved porcelain/human modes.

• Documentation

  • Major README overhaul plus new docs: Environment Variables, combined CLI reference, technical architecture, operations, and CLI command consolidation.

• Tests

  • Added URL-safety unit test; E2E runner now accepts an external binary or discovers local builds.

• CI

  • Added Markdown linting, a best-effort dev-shell warm step, and E2E execution inside the CI Docker image.

• Chores

  • Pre-commit and Makefile targets plus markdownlint config for repo Markdown linting.