Fix zensical install instructions: pip → pipx

[v0lkan]

Problem

All zensical install instructions say pip install zensical, which fails on modern macOS:

1. System Python 3.9 (macOS Sequoia): installs a stub package v0.0.2 that has no CLI binary — zensical requires Python >= 3.10
2. Homebrew Python 3.12+: blocked by PEP 668 (externally-managed-environment) — Homebrew refuses system-wide pip installs
3. No pip on PATH: many macOS setups only have pip3, so pip install itself fails

The working install path is:

pipx install zensical

pipx creates an isolated venv with whatever Python is available (even grabs 3.14 via Homebrew if needed), installs the package, and symlinks the binary to ~/.local/bin/.

Related: #6 (original report of the installation failure)

Affected Locations

User-facing docs (5 files)

File	Line	Current	Suggested
docs/session-journal.md	472	pip install zensical	pipx install zensical
docs/cli-reference.md	685	pip install zensical	pipx install zensical
docs/cli-reference.md	712	pip install zensical	pipx install zensical
docs/recipes/publishing.md	153	(pip install zensical)	(pipx install zensical)
docs/recipes/session-archaeology.md	265	(pip install zensical)	(pipx install zensical)

Go source — error messages (4 locations)

File	Line	Current	Suggested
internal/cli/journal/err.go	93	"zensical not found. Install with: pip install zensical"	"zensical not found. Install with: pipx install zensical"
internal/cli/serve/err.go	49	"zensical not found. Install with: pip install zensical"	"zensical not found. Install with: pipx install zensical"
internal/cli/journal/site.go	39	pip install zensical (in help text)	pipx install zensical
internal/cli/serve/serve.go	28	pip install zensical (in help text)	pipx install zensical

Makefile

File	Line	Current	Suggested
Makefile	151	.venv/bin/pip install zensical	This one is fine as-is — it installs inside a venv, so pip works correctly here. Just add a comment noting the venv context.

Suggested Approach

1. Replace pip install zensical → pipx install zensical in all docs and error messages listed above
2. Keep the Makefile's .venv/bin/pip install zensical since it's inside a venv (pip is correct there)
3. Consider adding a note in docs/session-journal.md about the Python >= 3.10 requirement and the macOS gotcha:

   > **macOS note:** `pip install zensical` may install a non-functional stub on
   > system Python 3.9. Use `pipx install zensical` instead, which handles
   > Python version management automatically.
   

4. Optionally, the Go code could auto-detect the platform and suggest brew install pipx && pipx install zensical on macOS

Environment

• macOS 15.3.2 (Sequoia), Apple Silicon
• System Python: 3.9.6
• Homebrew Python: 3.12, 3.14
• Working install: pipx install zensical → v0.0.21