Feat/explicit context dir

[josealekhine]

feat: enforce explicit CTX_DIR with single-source anchor resolution

Replaces directory walk-up with explicit declaration via CTX_DIR
(canonical-basename and absolute-path enforced). The anchor now
arrives through ctx activate / ctx deactivate shell hooks plus
per-tool channels; non-exempt commands refuse to run without it.
ContextChild and the implicit walk-up paths are removed; over
twenty command entry points migrated through rc.RequireContextDir.

Highlights:

• New cli/activate, cli/deactivate, system/check-anchor-drift
    commands plus the shell-emit primitives in config/shell and
    write/activate.
• rc.ContextDir and state.Dir tightened to (string, error) with
    ErrDirNotDeclared. Callers now gate on err, eliminating the
    filepath.Join("", rel) trap that wrote state files into CWD.
• testutil/testctx package introduced. Declare wires CTX_DIR and
    HOME for hermetic tests, eliminating cross-package races on
    ~/.claude/settings.json under parallel go test ./...
• Test isolation: fourteen test files now declare CTX_DIR and
    HOME so a developer shell with CTX_DIR exported no longer
    pollutes test runs. cli_test.go's withCtxEnv helper replaced
    with a single t.Setenv at the top of TestBinaryIntegration.
• Block-dangerous-command subtree retired in favor of the
    block-dangerous-commands plural canonical naming.
• examples/demo/ removed (superseded by docs runbooks).
• .context/ updated end-to-end: agent playbook, conventions,
    decisions, learnings, glossary, architecture, plus four new
    steering files.