docs: rewrite README, TODO, AGENTS and CONVENTIONS to theory-only scope

Compactify the descriptive markdown: scope statement replaces the
g_V-extraction goal, package architecture describes the single
theory-only distributable, roadmap keeps calculator capabilities
(C1-C8) and drops the analysis tracks, structure trees match the
actual file layout, the bs_exp debug step and detector-parameter
convention wording are dropped.

AGENTS.md

@@ -1,6 +1,6 @@
 # Agent Instructions — beta-spectrum
 
-High-precision beta decay spectrum calculator. Extracts shape factor C(W) from ⁹⁹Tc beta spectrum to parametrize g_V coupling.
+High-precision beta decay spectrum shape calculator. Since v0.5.0 the repository is dedicated solely to spectrum shape calculation: detector response, calibration, fitting and C(W)/g_V/g_A extraction live in separate, dedicated projects.
 
 ## Stack & Commands
 
@@ -68,7 +68,6 @@ After quality gates pass, run:
 
 ```bash
 bs_pnpi --nuclide Tc99 -vv --output ./output/verification.csv --plot ./output/verification.png --log-file ./output/debug.log 2>&1 | tee ./output/debug_stdout.log
-bs_exp --help                    # verify experimental data CLI
 ```
 
 Verify parameter consistency across all components. Save artifacts to `./output/`.