config: rename [paths]/[reports.*] sections to singular

config: rename [paths]/[reports.*] sections to singular

Every config section family is singular (thread, throttle, log, state,
scan, filter, hash, rewrite, tempfile, loop) except [paths] and
[reports.*].  The v1 schema is still unpublished, so this is the last
chance to make section names consistent without a compatibility alias or
version bump: rename [paths] -> [path] and [reports.*] -> [report.*].

Only the section token changes, not keys: paths.home -> path.home,
paths.reports -> path.reports (the "reports" key naming the report
directory is unchanged), and reports.<name> -> report.<name>.  The
reporter section enumeration in bees_main is updated to sections("report")
with the matching "report." prefix; bees-context.cc, bees-usage.cc, and
the BeesReportConfig doc comment follow.

While renaming bees-usage.cc, fix a stale key reference: the BEESHOME
environment variable was documented as [paths.beeshome], but the key is
home (".beeshome" is only its default value), so it now reads [path.home].

Docs (config-file.md, running.md, wrong.md) are updated in lockstep;
English prose using "reports"/"paths" as ordinary words is left intact.

Assisted-by: Claude-Code:claude-opus-4-8